[css-typed-om] Add Serializable attribute to css-typed-om objects

[DevSDK]

Fixes #1101

[w3cbot]

svgeesus marked as non substantive for IPR from ash-nazg.

[DevSDK]

I think the base class that cannot be constructed should not be marked as Serializable e.g., CSSMathValue due to they seems not have instance itself. (please correct me if I missed something)

On the other hand, the spec for FileSystemDirectoryHandle [1] marks the parent interface as Serializable, which also makes sense.

So my question is: is there a general rule here? For example, if a subinterface is Serializable, must the parent interface also be Serializable?

[1] https://fs.spec.whatwg.org/#api-filesystemdirectoryhandle