This refactors the dynamic check of which implementations are supported at runtime.
It also reduces duplicated effort in the CI fuzzing job, the differential fuzzers don't need to run with different values of SIMDJSON_FORCE_IMPLEMENTATION.
There is also a convenience script to run the fuzzers locally, to quickly check that the fuzzers still build, run and no easy to find bugs are there. It should be handy not only when developing the fuzzers, but also when modifying simdjson.
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