Return find_references results to caller

[metaleap]

This is so that users can optionally hook up another handling than ui._print to find_references results, yet the change is non-breaking. (So plain old normal lsp.find_references() calls see no change in behavior.)

(If you're curious, with this PR accepted, then in my case I'd later implement in my init.lua a ui.dialogs.filteredlist-based handling of find_references results. =)

[orbitalquark]

Thanks for the suggestion. I'm not sure how I feel about exposing implementation-specific details to user scripts. In this case, `find_references()` would be returning a table (or nil) of locations, which are composed of `uri` and then `range` tables with its own structure. I know these structures are documented in the LSP and probably will not change, but it doesn't feel right to forward that to the user. I think perhaps having a module option (e.g. `find_references_dialog = true`) and then changing the function to either print or show a filteredlist dialog is a better way to go about this. I'm not sure how you'd display the results in the filteredlist dialog though, especially when it comes to searching. Do you have any ideas?