Add ways to modify the output executable name

[fire]

Godot version

v4.4

godot-cpp version

https://github.com/godotengine/godot-cpp/tree/fbbf9ec4efd8f1055d00edb8d926eef8ba4c2cce

System information

Windows 11

Issue description

Document ways to modify the output gdextension library name without hacking the SCsub.

[dsnopek]

Document ways to modify the output gdextension library name without hacking the SCsub.

I feel like modifying the SConstruct is the right way to change the library name, though. This isn't something the user needs to customize - a particular project is going to use a particular hard-coded name.