refactor(lsp): move completion logic into _completion module

To reduce cross-chatter between modules and for https://github.com/neovim/neovim/issues/25272
Also preparing for https://github.com/neovim/neovim/issues/25714

runtime/lua/vim/lsp/protocol.lua

@@ -242,6 +242,7 @@ local constants = {
 
   -- Defines whether the insert text in a completion item should be interpreted as
   -- plain text or a snippet.
+  --- @enum lsp.InsertTextFormat
   InsertTextFormat = {
     -- The primary text to be inserted is treated as a plain string.
     PlainText = 1,