RenameProvider should be able to signal whether a location is valid for a rename

[DanTup]

Currently a RenameProvider can be instructed to make renames that are not valid, eg.:

import "dart:html";

Here, you can rename on the import which makes no sense. While it's possible for us to return no edits, it's weird to give the user a textbox pre-populated with import.

It would be better if a RenameProvider can be queried to see whether a rename is valid for a given location and Code can then avoid showing the rename box entirely.

[jrieken]

fyi @aeschli

[jrieken]

closing this as a duplicate of #7340