feat(lsp): builtin :lsp command

Problem:
- Despite [nvim-lspconfig](https://github.com/neovim/nvim-lspconfig)
  claims to be a "data-only" plugin, in fact it still provides some
  user-facing commands because they haven't been upstreamed to Nvim.

Solution:
- Upstream `:LspRestart`, `:LspStart` and `:LspStop` commands as `:lsp
  restart`, `:lsp start` and `:lsp stop` respectively.

Co-authored-by: glepnir <glephunter@gmail.com>

runtime/lua/vim/lsp/_cmd.lua

@@ -0,0 +1,142 @@
+local lsp = vim.lsp
+
+local M = {}
+
+--- @param filter? vim.lsp.get_clients.Filter
+--- @return string[]
+local function get_client_names(filter)
+  return vim
+    .iter(lsp.get_clients(filter))
+    :map(function(client)
+      return client.name
+    end)
+    :filter(function(name)
+      return vim.lsp.config[name] ~= nil
+    end)
+    :totable()
+end
+
+---@return string[]
+local function get_config_names()
+  local config_names = vim
+    .iter(vim.api.nvim_get_runtime_file('lsp/*.lua', true))
+    ---@param path string
+    :map(function(path)
+      local file_name = path:match('[^/]*.lua$')
+      return file_name:sub(0, #file_name - 4)
+    end)
+    :totable()
+
+  ---@diagnostic disable-next-line: invisible
+  vim.list_extend(config_names, vim.tbl_keys(vim.lsp.config._configs))
+  return vim.list.unique(config_names)
+end
+
+local complete_args = {
+  start = get_config_names,
+  stop = get_client_names,
+  restart = get_client_names,
+}
+
+local function ex_lsp_start(servers)
+  -- Default to enabling all servers matching the filetype of the current buffer.
+  -- This assumes that they've been explicitly configured through `vim.lsp.config`,
+  -- otherwise they won't be present in the private `vim.lsp.config._configs` table.
+  if #servers == 0 then
+    local filetype = vim.bo.filetype
+    ---@diagnostic disable-next-line: invisible
+    for name, _ in pairs(vim.lsp.config._configs) do
+      local filetypes = vim.lsp.config[name].filetypes
+      if filetypes and vim.tbl_contains(filetypes, filetype) then
+        table.insert(servers, name)
+      end
+    end
+  end
+
+  vim.lsp.enable(servers)
+end
+
+---@param clients string[]
+local function ex_lsp_stop(clients)
+  -- Default to disabling all servers on current buffer
+  if #clients == 0 then
+    clients = get_client_names { bufnr = vim.api.nvim_get_current_buf() }
+  end
+
+  for _, name in ipairs(clients) do
+    if vim.lsp.config[name] == nil then
+      vim.notify(("Invalid server name '%s'"):format(name))
+    else
+      vim.lsp.enable(name, false)
+    end
+  end
+end
+
+---@param clients string[]
+local function ex_lsp_restart(clients)
+  -- Default to restarting all active servers
+  if #clients == 0 then
+    clients = get_client_names()
+  end
+
+  for _, name in ipairs(clients) do
+    if vim.lsp.config[name] == nil then
+      vim.notify(("Invalid server name '%s'"):format(name))
+    else
+      vim.lsp.enable(name, false)
+    end
+  end
+
+  local timer = assert(vim.uv.new_timer())
+  timer:start(500, 0, function()
+    for _, name in ipairs(clients) do
+      vim.schedule_wrap(function(x)
+        vim.lsp.enable(x)
+      end)(name)
+    end
+  end)
+end
+
+local actions = {
+  start = ex_lsp_start,
+  restart = ex_lsp_restart,
+  stop = ex_lsp_stop,
+}
+
+local available_subcmds = vim.tbl_keys(actions)
+
+--- Use for `:lsp {subcmd} {clients}` command
+---@param args string
+M._ex_lsp = function(args)
+  local fargs = vim.api.nvim_parse_cmd('lsp ' .. args, {}).args
+  if not fargs then
+    return
+  end
+  local subcmd = fargs[1]
+  if not vim.list_contains(available_subcmds, subcmd) then
+    vim.notify(("Invalid subcommand '%s'"):format(subcmd), vim.log.levels.ERROR)
+    return
+  end
+
+  local clients = { unpack(fargs, 2) }
+
+  actions[subcmd](clients)
+end
+
+--- Completion logic for `:lsp` command
+--- @param line string content of the current command line
+--- @return string[] list of completions
+function M._ex_lsp_complete(line)
+  local splited = vim.split(line, '%s+')
+  if #splited == 2 then
+    return available_subcmds
+  else
+    local subcmd = splited[2]
+    ---@param n string
+    return vim.tbl_map(function(n)
+      return vim.fn.escape(n, [[" |]])
+    end, complete_args[subcmd]())
+  end
+end
+
+return M