Rework `Constraint` type

[arcnmx]

By using enums and specific types for the supported primitives, Constraint could be represented better. It's a bit messy because some constraints take a Number | Boolean, others String | Number, and then there's RangeInclusive<Number>...

~~Since the glib-serde dep is currently only used by Constraint's serde impls, so it could be removed as a core dep!~~ (this now uses LuaVariant, which it won't need anymore once the above rework is completed)