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()`.

src/nvim/options.lua

@@ -6755,6 +6755,7 @@ return {
           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|