LibJS: Save and restore exceptions on yields in finalizers

[Hendiadyoin1]

Also removes a bunch of related old FIXMEs.

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Test262:

    test/built-ins/AsyncGeneratorPrototype/throw/throw-suspendedYield-try-finally.js                       ❌ -> ✅
    test/built-ins/GeneratorPrototype/return/try-finally-nested-try-catch-within-inner-try.js              ❌ -> ✅
    test/built-ins/GeneratorPrototype/throw/try-finally-nested-try-catch-within-catch.js                   ❌ -> ✅
    test/built-ins/GeneratorPrototype/throw/try-finally-nested-try-catch-within-outer-try-after-nested.js  ❌ -> ✅
    test/built-ins/GeneratorPrototype/throw/try-finally-nested-try-catch-within-outer-try-before-nested.js ❌ -> ✅
    test/built-ins/GeneratorPrototype/throw/try-finally-within-try.js                                      ❌ -> ✅