Fix interface handling to support renaming across interface files

Hi there, I'm using `reason-langauge-server` along with `vscode` and so far it's been a solid experience. When using symbol renaming (`F2`), it seems to only work in the scope of a single file / module. It would be so nice if renaming could be propagated to the entire project.

I know this feature isn't easy, so just want to put in a request to keep on the backlog : )

No implementation files or tests are named. Start by locating the symbol-renaming handler invoked by F2 and the code that processes interface files; determine how project-wide references are discovered. Done means a rename propagates consistently across the entire project, including interface files.

Written by the indexing model from the issue text.