Add a new dialect= argument to st.from_regex()

[Zac-HD]

Some time ago, we added a private helper (#3196) in order to support the jsonschema semantics of $.

I think it's worth making a public API for this, along with considerably stricter validation - i.e. ensure that if we disallow trailing newline for $, we also enforce that you're using only the recommended subset of regex syntax which is (with that constraint) identical between Python and Javascript.

(cc @Stranger6667 - if we make validation stricter, will that break you? I guess we could do a jsonschema-python-no-validation dialect if necessary, practicality beats purity...)

Interface: add a new keyword-only argument, from_regex(..., dialect: Literal["python", "jsonschema"] = "python"). Obviously this makes it easy - at the interface level, anyway - to support additional dialects in future, if e.g. we can convert them using something like qntm/greenery.