GDScript: Add "Quick Fix" to create function stub matching signal connection

[Meorge]

What problem(s) does this PR solve?

• Implements part of LSP: Add textDocument/codeAction for generating method stubs for missing functions godot-proposals#14307 (but does not close it completely)
  • This PR could be expanded to also create general function stubs, and then perhaps close the issue.

Additional information

Note

This PR is dependent on #121338 being merged first. It looks like the branch, and thus the PR itself, include #121338's commits in it so this could technically be merged on its own and it would supersede #121338. But I think it would be better to merge #121338 on its own first, then have these changes reviewed and merged separately.

This PR adds a "Quick Fix" code action, which detects when a non-existent function name is provided to a Signal.connect() call, and creates a function stub with the signal's signature below the current function.

Godot.-.CA.Signal.Stub.mp4

To Do

• Confirm signal detection works when the signal belongs to the script class (i.e., defined with signal some_name(...) up above in the file)
• Confirm signal detection works when the signal is from the class the script inherits from (i.e., script has extends SomeOtherClass)
• Move detection/construction of code actions from gdscript_analyzer.cpp into push_error(), perhaps by adding a "code action hint" optional parameter to push_error()
• Ensure a code action that aims to insert text at the end of the file creates space for it
• Create tests
• Respect type hint settings for created function arguments and return type

[Meorge]

Tests have been added!