Create object for NSID / Add Validation

[drasticactions]

Namespace Identifiers (NSIDs) are treated as strings, when they could be made into a hardcast object with validation. Originally, I kept it as strings since there were implementations in the wild that did not map to it, making an object not worth doing without risk of something exploding during normal operation, but now is probably a good time to make it an actual object with validation.

[blowdart]

Feel free to go appropriate my one, you'll need four files,

• Nsid.cs
• NsidFormatException
• NsidConverter.cs for json serialization
• InternalStringExtensions.cs

The tests live in NsidTests.cs

In theory the regex should work, and you could chop out everything in nsid.cs's Parse() after !s_validationRegex().IsMatch(s), but I never trust my own regex ability, so I added way more validation after, just in case.

If you think it's worth me moving all the AtProto types and json converters out into a separate nuget package so you could just reference let me know.