global: Enable static class blocks

Class static blocks have slight conformance issues around await in mozjs91. The potential for programs to introduce these syntax errors seems small and we'd gain full conformance in the next ESR so perhaps should consider enabling.

• https://github.com/tc39/test262/pull/2968#issuecomment-849763483
• https://bugzilla.mozilla.org/show_bug.cgi?id=1713155

We should not merge this before !700 (merged) as the static this will be different internally vs. externally