feat: neovim client-side check and configure completion callbacks

[Myriad-Dreamin]

This PR shows the way of setting up completion callbacks for the language server to enhance experience of the lsp-based completion. As #875 said, these commands cannot be checked at (language) server side, so the client must take responsibility to check them and push configuration to the server. todo list:

• check existence of commands, triggerSuggest and triggerParameterHints
• implement command tinymist.triggerSuggestAndParameterHints to callback above two commands.
• find a way to promote the configuration, e.g. add a PR to nvim-lspconfig.