Add a prototype addon that includes the formatter as a gdextension

[khellste]

Please check if the PR fulfills these requirements:

• The commit message follows our guidelines.
• For bug fixes and features:
  • You tested the changes.

Related issue (if applicable): #282

What kind of change does this PR introduce?
This PR introduces a gdextension build target for the formatter, allowing the formatter to run natively in the Godot editor rather than as a separate binary. It also adds a new "standalone" version of the addon that uses this gdextension instead of shelling out to the formatter binary.

Does this PR introduce a breaking change?
No.

New feature or change

What is the current behavior?
The Godot addon requires a separate formatter binary to be installed, and must call this binary in a separate process.

What is the new behavior?
The new Godot addon includes the formatter as a gdextension and runs the formatter natively in the Godot editor.

Other information

• Updating from the editor is not implemented in the forked addon.
• Documentation is not updated.
• I couldn't find any unit tests for the addon, so I only did manual testing.
• I couldn't find the commit message guidelines.

[NathanLovato]

Thank you very much for your work on this!

I just got around to testing it, just had to bump the rust version to get it to compile. Also changed an import path which looked like it was accessing data structures through a path it wasn't meant to; turns out there's a godot::builtin package to access Array etc.

[NathanLovato]

I haven't taken the time to look into next steps yet for this. I'm merging notably so we can test it more and so you know your work is in.