Prevent quoted args in editor/main_run_args from being split at spaces

[lpriebe]

When running from the editor with main_run_args set in Project Settings, said
arguments were being split at spaces even when enclosed in quotes. (This didn't
affect Windows, since the arguments were spliced back together anyway in
OS_Windows::create_process, but the OS_Unix equivalent can't do that)

This commit uses another method to split the arguments, which leaves quoted
strings intact.

Fixes #72935.

[Qubus0]

nice, thank you. any chance this might be backported to 3.5?

[Calinou]

Tested locally on Linux (rebased against master 7641936), it works.

Output from the MRP linked in #72935 with this PR:

["res://Main.tscn", "-arg=\"string with spaces\"", "--second-arg", "\'with spaces\'"]
single arg:	res://Main.tscn
single arg:	-arg="string with spaces"
single arg:	--second-arg
single arg:	'with spaces'
---
["res://Main.tscn", "-arg=\"string with spaces\"", "--second-arg", "\'with spaces\'"]
fixed arg:	res://Main.tscn
fixed arg:	-arg="string with spaces"
fixed arg:	--second-arg
fixed arg:	'with spaces'

I've also tested more involved scenarios with %command% and a engine binary path containing spaces, and it works as expected.

[akien-mga]

Thanks!