feat(lua): add vim.keymap

This introduces two new functions `vim.keymap.set` & `vim.keymap.del`

differences compared to regular set_keymap:
- remap is used as opposite of noremap. By default it's true for <Plug> keymaps and false for others.
- rhs can be lua function.
- mode can be a list of modes.
- replace_keycodes option for lua function expr maps. (Default: true)
- handles buffer specific keymaps

Examples:
```lua
vim.keymap.set('n', 'asdf', function() print("real lua function") end)
vim.keymap.set({'n', 'v'}, '<leader>lr', vim.lsp.buf.references, {buffer=true})
vim.keymap.set('n', '<leader>w', "<cmd>w<cr>", {silent = true, buffer = 5 })
vim.keymap.set('i', '<Tab>', function()
  return vim.fn.pumvisible() == 1 and "<C-n>" or "<Tab>"
end, {expr = true})
vim.keymap.set('n', '[%', '<Plug>(MatchitNormalMultiBackward)')

vim.keymap.del('n', 'asdf')
vim.keymap.del({'n', 'i', 'v'}, '<leader>w', {buffer = 5 })
```

runtime/lua/vim/keymap.lua

@@ -0,0 +1,135 @@
+local keymap = {}
+
+--- Add a new |mapping|.
+--- Examples:
+--- <pre>
+---   -- Can add mapping to Lua functions
+---   vim.keymap.set('n', 'lhs', function() print("real lua function") end)
+---
+---   -- Can use it to map multiple modes
+---   vim.keymap.set({'n', 'v'}, '<leader>lr', vim.lsp.buf.references, { buffer=true })
+---
+---   -- Can add mapping for specific buffer
+---   vim.keymap.set('n', '<leader>w', "<cmd>w<cr>", { silent = true, buffer = 5 })
+---
+---   -- Expr mappings
+---   vim.keymap.set('i', '<Tab>', function()
+---     return vim.fn.pumvisible() == 1 and "<C-n>" or "<Tab>"
+---   end, { expr = true })
+---   -- <Plug> mappings
+---   vim.keymap.set('n', '[%%', '<Plug>(MatchitNormalMultiBackward)')
+--- </pre>
+---
+--- Note that in a mapping like:
+--- <pre>
+---    vim.keymap.set('n', 'asdf', require('jkl').my_fun)
+--- </pre>
+---
+--- the require('jkl') gets evaluated during this call in order to access the function. If you want to
+--- avoid this cost at startup you can wrap it in a function, for example:
+--- <pre>
+---    vim.keymap.set('n', 'asdf', function() return require('jkl').my_fun() end)
+--- </pre>
+---
+---@param mode string|table   Same mode short names as |nvim_set_keymap()|.
+---                            Can also be list of modes to create mapping on multiple modes.
+---@param lhs string          Left-hand side |{lhs}| of the mapping.
+---@param rhs string|function  Right-hand side |{rhs}| of the mapping. Can also be a Lua function.
+--
+---@param opts table A table of |:map-arguments| such as "silent". In addition to the options
+---                  listed in |nvim_set_keymap()|, this table also accepts the following keys:
+---                  - replace_keycodes: (boolean, default true) When both this and expr is "true",
+---                  |nvim_replace_termcodes()| is applied to the result of Lua expr maps.
+---                  - remap: (boolean) Make the mapping recursive. This is the
+---                  inverse of the "noremap" option from |nvim_set_keymap()|.
+---                  Default `true` if `lhs` is a string starting with `<plug>` (case-insensitive), `false` otherwise.
+---@see |nvim_set_keymap()|
+function keymap.set(mode, lhs, rhs, opts)
+  vim.validate {
+    mode = {mode, {'s', 't'}},
+    lhs = {lhs, 's'},
+    rhs = {rhs, {'s', 'f'}},
+    opts = {opts, 't', true}
+  }
+
+  opts = vim.deepcopy(opts) or {}
+  local is_rhs_luaref = type(rhs) == "function"
+  mode = type(mode) == 'string' and {mode} or mode
+
+  if is_rhs_luaref and opts.expr and opts.replace_keycodes ~= false then
+    local user_rhs = rhs
+    rhs = function ()
+      return vim.api.nvim_replace_termcodes(user_rhs(), true, true, true)
+    end
+  end
+  -- clear replace_keycodes from opts table
+  opts.replace_keycodes = nil
+
+  if opts.remap == nil then
+    -- remap by default on <plug> mappings and don't otherwise.
+    opts.noremap = is_rhs_luaref or rhs:lower():match("^<plug>") == nil
+  else
+    -- remaps behavior is opposite of noremap option.
+    opts.noremap = not opts.remap
+    opts.remap = nil
+  end
+
+  if is_rhs_luaref then
+    opts.callback = rhs
+    rhs = ''
+  end
+
+  if opts.buffer then
+    local bufnr = opts.buffer == true and 0 or opts.buffer
+    opts.buffer = nil
+    for _, m in ipairs(mode) do
+      vim.api.nvim_buf_set_keymap(bufnr, m, lhs, rhs, opts)
+    end
+  else
+    opts.buffer = nil
+    for _, m in ipairs(mode) do
+      vim.api.nvim_set_keymap(m, lhs, rhs, opts)
+    end
+  end
+end
+
+--- Remove an existing mapping.
+--- Examples:
+--- <pre>
+---   vim.keymap.del('n', 'lhs')
+---
+---   vim.keymap.del({'n', 'i', 'v'}, '<leader>w', { buffer = 5 })
+--- </pre>
+---@param opts table A table of optional arguments:
+---                  - buffer: (number or boolean) Remove a mapping from the given buffer.
+---                  When "true" or 0, use the current buffer.
+---@see |vim.keymap.set()|
+---
+function keymap.del(modes, lhs, opts)
+  vim.validate {
+    mode = {modes, {'s', 't'}},
+    lhs = {lhs, 's'},
+    opts = {opts, 't', true}
+  }
+
+  opts = opts or {}
+  modes = type(modes) == 'string' and {modes} or modes
+
+  local buffer = false
+  if opts.buffer ~= nil then
+    buffer = opts.buffer == true and 0 or opts.buffer
+    opts.buffer = nil
+  end
+
+  if buffer == false then
+    for _, mode in ipairs(modes) do
+      vim.api.nvim_del_keymap(mode, lhs)
+    end
+  else
+    for _, mode in ipairs(modes) do
+      vim.api.nvim_buf_del_keymap(buffer, mode, lhs)
+    end
+  end
+end
+
+return keymap