Refactor ProjectSetting overrides

[reduz]

• Overrides no longer happen for set/get.
• They must be checked with a new function: ProjectSettings::get_setting_with_override().
• GLOBAL_DEF/GLOBAL_GET updated to use the above function.
• The ability to disable feature overrides has been removed since it no longer makes sense.

This change solves many problems:

• General confusion about getting the actual or overriden setting.
• Feature tags available after settings are loaded were being ignored, they are now considered.
• Hacks required for the Project Settings editor to work.

• Fixes Project settings feature overrides aren't applied in the editor #64100.
• Fixes up Only define keep_screen_on project setting once #64014 and Tweak default crash handler message in exported projects #61908.
• Supersedes Fix keep_screen_on being applied for the editor #71076.

[akien-mga]

There are also these usage of ProjectSettings::get_setting() which should likely be replaced by GLOBAL_GET() or get_setting_with_override() to properly respect overrides:

tests/core/config/test_project_settings.h
43:     Variant variant = ProjectSettings::get_singleton()->get_setting("application/config/name");
53:     Variant variant = ProjectSettings::get_singleton()->get_setting("application/config/name", "SomeDefaultValue");
63:     Variant variant = ProjectSettings::get_singleton()->get_setting("not_existing_setting");
70:     Variant variant = ProjectSettings::get_singleton()->get_setting("not_existing_setting");
73:     variant = ProjectSettings::get_singleton()->get_setting("not_existing_setting", "my_nice_default_value");
85:     Variant variant = ProjectSettings::get_singleton()->get_setting("my_custom_setting");
91:     variant = ProjectSettings::get_singleton()->get_setting("my_custom_setting");

modules/mono/csharp_script.cpp
691:            String assembly_name = ProjectSettings::get_singleton()->get_setting("dotnet/project/assembly_name");

modules/mono/mono_gd/gd_mono.cpp
292:    String assembly_name = ProjectSettings::get_singleton()->get_setting("dotnet/project/assembly_name");
469:    String assembly_name = ProjectSettings::get_singleton()->get_setting("dotnet/project/assembly_name");

platform/web/export/export_plugin.cpp
156:    replaces["$GODOT_PROJECT_NAME"] = ProjectSettings::get_singleton()->get_setting("application/config/name");
196:    String proj_name = ProjectSettings::get_singleton()->get_setting("application/config/name");

[reduz]

Removed function to disable, applied fixes and updated docs.

[akien-mga]

Thanks!