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.