Resolve `completionItem/resolve` request

[elianiva]

I don't know if this should be implemented in a completion plugin or upstream, but I'd love if rofl could resolve completionItem/resolve so it can auto-import stuff for me (it's tedious to import stuff manually and sometimes codeactions doesn't work). This is one of the reason why I like nvim-compe, it would be great if rofl.nvim also supports this :)