Add UnknownType and use in converters

[mjperrone]

Addresses https://github.com/recap-build/recap/issues/429

Adds a new type UnknownType for situations where converters can't figure out an appropriate existing recap type.

This does not have comprehensive test cases and I'm not sure this is the right way to go for all these converters, but I wanted to get started on this so we can discuss and collaborate on it.

Made a PR for the website/json schema update: https://github.com/recap-build/recap-website/pull/16

Validation

We will need to add a bunch of tests for the converters, parser, and for the JSON schema validation.

[criccomini]

Great, thanks! I'll take a look tomorrow.

One thing I noticed in the website PR was that it seems like you might be modifying the CST to add an "unknown" type. This was something I mentioned wanting to avoid in #429:

If we went this route, I wouldn't want to add an unknown to the CST, but rather keep UnknownType as only an AST class similar to ProxyType.

I'll take a look and see if we can get away without adding "unknown" as a concrete syntax.

[mjperrone]

Ah yes, forgot/missed that. Thanks for taking a look

[mjperrone]

I don't think I will be able to make another iteration on this for several more weeks

[criccomini]

No worries, there's nothing pressing on my end.