Add normalize parameter to Str::studly() and Str::pascal() (#60229)

Adds an optional `normalize: true` flag that lowercases all-uppercase
word segments before conversion, so acronym-style strings like "CBOR"
and "FMLS" produce "Cbor" and "Fmls" instead of being left unchanged.

Without the flag the behaviour is identical to today, so this is
fully backwards-compatible.