Allow colon in locale message keys used in linked locale messages

[ax-jmckenzie]

Linked Locale Messages break when the linked locale message key contains a : character. This updates a regex to allow for keys containing :, as long as the linked locale message uses the brackets syntax.

example:

{
  "homeAddress:phrase": "Home Address",
  "missingHomeAddress:validation": 'Please provide @.lower:(homeAddress:phrase)'
}

[kazupon]

Thank you for your contribution!

Could you add the unit test cases, please? 🙏
Our maintainers need to ensure that your updating works correctly.

[ax-jmckenzie]

@kazupon, thanks for taking a look at this PR! I've added unit tests as requested.

[kazupon]

Thanks!
Good for your contribution! :)