feat(lsp): add vim.lsp.config and vim.lsp.enable

Design goals/requirements:
- Default configuration of a server can be distributed across multiple sources.
  - And via RTP discovery.
- Default configuration can be specified for all servers.
- Configuration _can_ be project specific.

Solution:

- Two new API's:
  - `vim.lsp.config(name, cfg)`:
    - Used to define default configurations for servers of name.
    - Can be used like a table or called as a function.
    - Use `vim.lsp.confg('*', cfg)` to specify default config for all
      servers.
  - `vim.lsp.enable(name)`
    - Used to enable servers of name. Uses configuration defined
    via `vim.lsp.config()`.

runtime/lua/vim/_meta/options.lua

@@ -5010,6 +5010,7 @@ vim.go.ruf = vim.go.rulerformat
 ---   indent/	indent scripts `indent-expression`
 ---   keymap/	key mapping files `mbyte-keymap`
 ---   lang/		menu translations `:menutrans`
+---   lsp/		LSP client configurations `lsp-config`
 ---   lua/		`Lua` plugins
 ---   menu.vim	GUI menus `menu.vim`
 ---   pack/		packages `:packadd`