Adding and removing extended modifiers

I was wondering why it isn't possible to directly add or remove extended modifiers. The only way is to copy the set, add your modifier and set it again. This is currently done in #4038 and #4134, and I'm currently trying to figure out some more issues with implicit modifiers where this functionality would make things a lot easier.

Is there any specific reason why such methods don't exist? If it just wasn't considered as useful before, I'd be happy to open a pull request for that.

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Edit: Another useful API would be two static factory methods in `CtExtendedModifier`:
- `CtExtentedModifier.implicit(ModifierKind)`
- `CtExtendedModifier.explicit(ModifierKind)`

This would make it easier and more readable to work with extended modifiers.