Expand search scope for "Go to Definition"

[jhan276]

Is your feature request related to a problem? Please describe.
When a function exists in a different file (but still in the same project folder), pressing F12 ("Go to Definition") doesn't work as the error message "No definition found for <func_name>" pops up.

Describe the solution you'd like
Like RStudio, I would like "Go to Definition" expand the search scope to the existing project folder. If multiple occurence, show it as dropdown list so user can select.

Thanks for you awesome extension!

[renkun-ken]

All language services are acutally provided by languageserver.

This is a duplicate of REditorSupport/languageserver#20.