Implement Optional Chaining and Nullish coalescing Operator in QML

Now that both the Optional Chaining and Nullish Coalescing proposals reached stage 3 of the Ecma TC39 proposals process ( https://tc39.es/process-document/ ), what about implementing them?

Those are being implemented in TypeScript ( see https://github.com/microsoft/TypeScript/issues/16 && https://github.com/microsoft/TypeScript/pull/32883 ) meaning they are confident the proposals won't really change at this point (cf. https://github.com/microsoft/TypeScript/issues/33118 : "We hold a policy of not implementing features unless they reach stage 3 and we have high confidence in them." )

Both those new syntaxes will enable more expressive and declarative code in QML.