Implement SetUnhandledExceptionHandler on Mono

[VSadov]

Re: #109806

Right now the API is only implemented/tested on CoreCLR and NativeAOT.
The testcases in baseservices/exceptions/UnhandledExceptionHandler are disabled for Mono.

Note - "Implement" could really throw NotImplemented exception, for example, but there need to be some implementation.

[VSadov]

The parts of the feature that affect threadpool threads might be already "done" as threadpool implementation is shared. (need to verify)

The part that affects exceptions thrown in user-launched threads may be "done" as well. If that part is shared. (need to verify)

The part that is in Finalizer thread, however, is definitely not shared with other runtimes and will need additional mono-specific implementation.

[steveisok]

Appears fixed in #118563

@VSadov if there's more, feel free to re-open.