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 })
```

scripts/gen_vimdoc.py

@@ -129,6 +129,7 @@ CONFIG = {
             'uri.lua',
             'ui.lua',
             'filetype.lua',
+            'keymap.lua',
         ],
         'files': ' '.join([
             os.path.join(base_dir, 'src/nvim/lua/vim.lua'),
@@ -136,6 +137,7 @@ CONFIG = {
             os.path.join(base_dir, 'runtime/lua/vim/uri.lua'),
             os.path.join(base_dir, 'runtime/lua/vim/ui.lua'),
             os.path.join(base_dir, 'runtime/lua/vim/filetype.lua'),
+            os.path.join(base_dir, 'runtime/lua/vim/keymap.lua'),
         ]),
         'file_patterns': '*.lua',
         'fn_name_prefix': '',
@@ -151,6 +153,7 @@ CONFIG = {
             'uri': 'vim',
             'ui': 'vim.ui',
             'filetype': 'vim.filetype',
+            'keymap': 'vim.keymap',
         },
         'append_only': [
             'shared.lua',