`<key> in <ClassRef>` reports false for static data props and `prototype`

[proggeramlug]

Summary

<key> in <ClassRef> reports false for a class's own static data properties
and for "prototype".

Repro

class K { static s = 1; m() {} }
console.log("s" in K);          // node: true   perry: false
console.log("prototype" in K);  // node: true   perry: false

Notes

Pre-existing (reproduces on main). A class used as a value is INT32-tagged; the
class-ref arm in js_object_has_property checks CLASS_STATIC_SYMBOLS for
symbol keys but has no string-key path for CLASS_DYNAMIC_PROPS / static data
fields (the key_val.is_string() branch is a no-op) and returns false.
Separate from the Object.create in fix (#6145).