Convert between Locale and String

### Use case

MapStruct should convert between `Locale` and `String` using `toLanguageTag()` and `forLanguageTag()`. While `Locale` has a `toString()` it does not have a way to parse the result so language tags are a better string version of a `Locale`.

I'll have a PR shortly.

### Generated Code

_No response_

### Possible workarounds

_No response_

### MapStruct Version

_No response_