Repository: godot
Issue: Unimplemented `Script::inherits_script` in custom modules break compilation for languages in 3.x
Labels: discussion,topic:core,breaks compat

### Godot version

3.4 beta

### System information

Windows 10

### Issue description

#51166 cause compilation errors for modules that implement new languages for Godot 3.x, which is compatibility breakage. Authors must have implement `Script::inherits_script` for their modules to work.

For instance:

```
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(493): error C2259: 'MixinScript': cannot instantiate abstract class
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.h(57): note: see declaration of 'MixinScript'
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(493): note: due to following members:
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(493): note: 'bool Script::inherits_script(const Ref<Script> &) const': is abstract
D:\usr\src\goostengine\goost\godot\core/script_language.h(122): note: see declaration of 'Script::inherits_script'
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(493): error C2259: 'MixinScript': cannot instantiate abstract class
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.h(57): note: see declaration of 'MixinScript'
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(493): note: due to following members:
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(493): note: 'bool Script::inherits_script(const Ref<Script> &) const': is abstract
D:\usr\src\goostengine\goost\godot\core/script_language.h(122): note: see declaration of 'Script::inherits_script'
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(499): error C2259: 'MixinScript': cannot instantiate abstract class
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.h(57): note: see declaration of 'MixinScript'
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(499): note: due to following members:
D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.cpp(499): note: 'bool Script::inherits_script(const Ref<Script> &) const': is abstract
D:\usr\src\goostengine\goost\godot\core/script_language.h(122): note: see declaration of 'Script::inherits_script'
scons: *** [D:\usr\src\goostengine\goost\core\script\mixin_script\mixin_script.windows.tools.64.goost.obj] Error 2
D:\usr\src\goostengine\goost\godot\core/class_db.h(140): error C2259: 'MixinScript': cannot instantiate abstract class
D:\usr\src\goostengine\goost\core\script\mixin_script\editor\../mixin_script.h(57): note: see declaration of 'MixinScript'
D:\usr\src\goostengine\goost\godot\core/class_db.h(140): note: due to following members:
D:\usr\src\goostengine\goost\godot\core/class_db.h(140): note: 'bool Script::inherits_script(const Ref<Script> &) const': is abstract
D:\usr\src\goostengine\goost\godot\core/script_language.h(122): note: see declaration of 'Script::inherits_script'
D:\usr\src\goostengine\goost\godot\core/class_db.h(179): note: see reference to function template instantiation 'Object *ClassDB::creator<T>(void)' being compiled
        with
        [
            T=MixinScript
        ]
D:\usr\src\goostengine\goost\core\script\register_script_types.cpp(34): note: see reference to function template instantiation 'void ClassDB::register_class<MixinScript>(void)' being compiled
D:\usr\src\goostengine\goost\godot\core/class_db.h(140): error C2259: 'MixinScript': cannot instantiate abstract class
D:\usr\src\goostengine\goost\core\script\mixin_script\editor\../mixin_script.h(57): note: see declaration of 'MixinScript'
D:\usr\src\goostengine\goost\godot\core/class_db.h(140): note: due to following members:
D:\usr\src\goostengine\goost\godot\core/class_db.h(140): note: 'bool Script::inherits_script(const Ref<Script> &) const': is abstract
D:\usr\src\goostengine\goost\godot\core/script_language.h(122): note: see declaration of 'Script::inherits_script'
scons: *** [D:\usr\src\goostengine\goost\core\script\register_script_types.windows.tools.64.goost.obj] Error 2
scons: building terminated because of errors.
```

If compatibility breakage is intended, it should be denoted in the `CHANGELOG.md`.

Shouldn't it be possible to provide default implementation?

### Steps to reproduce

~~Try to compile https://github.com/goostengine/goost using Godot 3.x.~~ Should be fixed in https://github.com/goostengine/goost/commit/d76d00d37840b7090fbdeff1506adb37c8bf1101.

### Minimal reproduction project

N/A