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()`.

scripts/gen_vimdoc.lua

@@ -515,6 +515,8 @@ local function inline_type(obj, classes)
   elseif desc == '' then
     if ty_islist then
       desc = desc .. 'A list of objects with the following fields:'
+    elseif cls.parent then
+      desc = desc .. fmt('Extends |%s| with the additional fields:', cls.parent)
     else
       desc = desc .. 'A table with the following fields:'
     end