Open missing source file errors on GitHub

[KoBeWi]

Follow-up to #111805

That PR didn't properly handle missing files. This PR makes the editor try to open them on GitHub.

It would be helpful if official builds came with some prefix for source paths, it would be easier to differentiate them from actually missing files in custom builds. The way I did it might be unreliable and try to open non-existent files.

[Calinou]

Tested locally, it doesn't seem to work on my end:

• If I keep the compiled binary in the source repository, it will open the source file in the external editor as intended.
• If I move the compiled binary out of the source repository, it will create a new file named after the last component of the path in the external editor (e.g. variant_utility.cpp for an error in ./core/variant/variant_utility.cpp:1024).

This presumably occurs because the path.begins_with("./") check applies even if the file doesn't exist.

Tested using this code (save it and use Scene > Reload Saved Scene, or restart the editor):

@tool
extends Node

func _ready() -> void:
	push_error("test")

[KoBeWi]

Should be fixed now.
Non-existent absolute paths will result in opening invalid GitHub page though, not sure if it can be fixed easily.

[YeldhamDev]

Works properly on my end. And I think it's fine to merge now, since currently clicking on the file links opens a non-existing webpage.

[Repiteo]

Thanks!