refactor: create function for deferred loading

The benefit of this is that users only pay for what they use. If e.g.
only `vim.lsp.buf_get_clients()` is called then they don't need to load
all modules under `vim.lsp` which could lead to significant startuptime
saving.

Also `vim.lsp.module` is a bit nicer to user compared to
`require("vim.lsp.module")`.

This isn't used for some nested modules such as `filetype` as it breaks
tests with error messages such as "attempt to index field 'detect'".
It's not entirely certain the reason for this, but it is likely it is
due to filetype being precompiled which would imply deferred loading
isn't needed for performance reasons.

runtime/lua/vim/lsp/util.lua

@@ -3,7 +3,7 @@ local snippet = require('vim.lsp._snippet_grammar')
 local validate = vim.validate
 local api = vim.api
 local list_extend = vim.list_extend
-local highlight = require('vim.highlight')
+local highlight = vim.highlight
 local uv = vim.uv
 
 local npcall = vim.F.npcall
@@ -636,7 +636,7 @@ end
 ---@see complete-items
 function M.text_document_completion_list_to_complete_items(result, prefix)
   vim.deprecate('vim.lsp.util.text_document_completion_list_to_complete_items()', nil, '0.11')
-  return require('vim.lsp._completion')._lsp_to_complete_items(result, prefix)
+  return vim.lsp._completion._lsp_to_complete_items(result, prefix)
 end
 
 --- Like vim.fn.bufwinid except it works across tabpages.