Change definition of ClassProp

[Austaras]

Change key field to PropName and remove computed. There's two reasons:

1. ClassProp key and ClassMethod key are both ClassElementName in spec, it's better to unify them
   
   
2. Current definition may generate invalid state. When computed is false, key could only be Number String BigInt

Do we need to wait for a new major release to fix this?

[kdy1]

No, I think it's fine.
AST cannot be totally stabilized anyway. I'll mark the change as a breaking change for rust side.