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/shared.lua

@@ -1409,4 +1409,14 @@ function vim._resolve_bufnr(bufnr)
   return bufnr
 end
 
+--- @generic T
+--- @param x elem_or_list<T>?
+--- @return T[]
+function vim._ensure_list(x)
+  if type(x) == 'table' then
+    return x
+  end
+  return { x }
+end
+
 return vim