getSHUnitCreator() to IHermes (#53419)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53419

By default, this function returns nullptr. User can pass a preprocessor
definition "-DHERMES_SH_UNIT_FN=sh_export_<unit_name>" (where
<unit_name> is the name passed to shermesc when compiling the JS
input), so that this function returns the function pointer, which can
be passed to `evaluateSHUnit` for evaluation.

Changelog: [Internal]

Reviewed By: avp

Differential Revision: D80747463

fbshipit-source-id: a798a7a572679444fca111c34674fd7ced9311f3