Improve `Locale` handling in `JsonFormat.Value`

[cowtowncoder]

(note: from FasterXML/jackson-databind#3316)

Currently handling of @JsonFormat.locale is sub-optimal: Locale is constructed in constructor of JsonFormat.Value, with:

new Locale(localeStr)

which works for simple language case like "de" (for German) or "en" (for English) but not for variants like "de_DE" / "de-DE" or "en_US".

Handling should be improved by using something like what databind method "_deserializeLocale()" in class JDKFromStringDeserializer does.

[cowtowncoder]

Implemented for 2.22.