Add basic Goto Implementation Request support

[smheidrich]

Description

As pointed out in #97, support for the LSP Goto Implementation Request (textDocument/implementation) aka implementationProvider capability is still missing.

This PR adds basic support for this feature using Rope's rope.findit.find_implementations function (so it only works if the rope extra is installed).

Rope's find_implementations function seems a bit brittle, e.g. it doesn't appear to work for some slightly more complicated inheritance hierarchies (multiple levels & multiple inheritance & generics, but not sure which of these is the issue), but these things should be fixed upstream & in any case it's better than nothing.

Status

• Blocked by:
  • findit.find_implementations on method call finds implementation for different subclass python-rope/rope#812
• Also at least 1 bug remaining on this side of the implementation:
  • Add basic Goto Implementation Request support #644 (comment)