Add support for built-string format checks

[jcockhren]

https://json-schema.org/understanding-json-schema/reference/string.html#built-in-formats

[jcockhren]

On second thought, string format checks may be a bridge too far (at least not as a core feature):

• Natively implementing multiple RFCs is a large undertaking and could be a project in and of itself.
• Format-specific error messages would require a complete re-work of error handling.
• A quick implementation would require new 3rd party dependencies. e.g., uritools or rfc3987

Given the availability of pre-existing libraries useful for constructing, parsing, and validating URI strings, this feature would be best served as an "extra" with (well-supported) optional dependencies. See commit 4eb1db5