fix inference for JTDSchemaType

[erikbrinkman]

What issue does this pull request resolve?
AJV required manually annotating compile and validate with the type to get inference to work. That's no longer the case.

What changes did you make?
I made two changes:

1. I split the overload declarations into separate declarations instead of unions
2. I upgraded typescript to 4.2

Both of these seem to be necessary. You can test in this playground, where removing either results in failed inference. However, I'm not sure why. I couldn't find much information on how the inference is done, and certainly not a spec that says when it works and when it doesn't. That aspect makes it seem like this behavior isn't great to rely on, but I guess as long as it works in test. I also scanned over the changes for typescript 4.2, but didn't see anything that indicated it would help with inference.

Is there anything that requires more attention while reviewing?
All of the tests pass, but this does upgrade typescript to 4.2, which maybe you don't want to do yet. Note, es-lint warns on typescript 4.1 and above, so that's not great, but it still works.

[epoberezkin]

Great - I would have never figured it out! Thank you.

I will add the same to compileParser and compileSerializer.

[epoberezkin]

Although need to check first - maybe inference works there, it's only for JTD so a bit simpler...

[epoberezkin]

yes, didn't work without splitting in the same way, even though there was no JSONSchemaType - added