Introduce a concrete result type for JSValue -> Implementation conversions

[Sam Weinig]

Introduce a concrete result type for JSValue -> Implementation conversions.

This is a precursor to supporting non-default initializable implementation types in a general way.

[Radar WebKit Bug Importer]

<rdar://problem/128703953>

[Sam Weinig]

Pull request: https://github.com/WebKit/WebKit/pull/29085

[EWS]

Committed 279321@main (9a160ff054ee): <https://commits.webkit.org/279321@main>

Reviewed commits have been landed. Closing PR #29085 and removing active labels.

[Fujii Hironori]

Regressed: Bug 274732 – REGRESSION(279321@main): js/dom/missing-exception-check-in-convertVariadicArguments.html is crashing : Unchecked exception detected at JSC::VM::verifyExceptionCheckNeedIsSatisfied : ASSERTION FAILED: !m_needExceptionCheck

[Diego Pino]

Build bot 'WPE-Linux-64-bit-Release-Ubuntu-2204-Build' is failing with the following build error after this patch:

https://build.webkit.org/#/builders/1060/builds/444

```
/home/buildbot/worker/WPE-Linux-64-bit-Release-Ubuntu-2204-Build/build/Source/WebCore/bindings/js/JSDOMConvertResult.h:119:60: error: could not convert ‘T{other.releaseReturnValue()}’ from ‘T’ to ‘WebCore::Detail::ConversionResultStorage<T&>::ReturnType’
  119 |             return ReturnType { other.releaseReturnValue() };
      |                                                            ^
      |                                                            |
      |                                                            T 
```

[Diego Pino]

Re-opening for pull request https://github.com/webkit/webkit/pull/29147

[EWS]

Committed 279350@main (cd96a999407c): <https://commits.webkit.org/279350@main>

Reviewed commits have been landed. Closing PR #29147 and removing active labels.