[css-fonts] [palettes] Properties in CSSFontPaletteValuesRule don't need to be mutable

[litherum]

Making the properties mutable adds significant implementation complexity because of shadow dom. For example, if you add the same sheet to two different shadow roots, and then one of them modifies it, it shouldn't appear modified from the other shadow root.

I can't actually think of a situation where anyone would actually want to modify the attributes in CSSFontPaletteValuesRule from script, rather than replacing the whole rule. We should just modify the attributes to be readonly.

[litherum]

Pull request: #6654