Deprecate damping, add reflectivity to match AudioEffectReverb's documentation

[PhairZ]

Closes #98945.
Renamed damping to reflectivity in the documentation and class of AudioEffectReverb. And tested it.

[akien-mga]

Changing the name of a property and its methods breaks compatibility, so this isn't something we can do so lightly.

If it's correct that the name damping is wrong, then renaming it to reflectivity may be correct, but a deprecated compatibility binding still needs to be included, so that user projects don't break.

[PhairZ]

This is the best I could do. I can't really wrap my head around compatibility issues.

[akien-mga]

You did pretty well! This would be sufficient if our compatibility system was advanced enough, but sadly that provides compatibility for GDExtension but not for GDScript. For GDScript we'll want to keep registering normal (not compatibility) set_damping/get_damping/damping methods/properties, just enclosed in #ifndef DISABLE_DEPRECATED.

The compatibility method bindings are used when doing changes like adding a new optional parameter, which breaks compat for GDExtension but not for GDScript.

I can update the PR myself with these changes if you want, it's not something we have well documented currently.

[PhairZ]

I'll try to give it my best later. I think i can handle it.

[Mickeon]

For reference, take a look at how Control's deprecated auto_translate property handles it. See also #87530

[PhairZ]

Thanks, @Mickeon. The deprecated 'auto_translate' property were a good reference. I'm glad it was able to pass CI.

[Mickeon]

I'm going to let someone else review the compatibility code