feat: Support constant type

[joshuat]

This is useful for creating discriminated unions with the oneOf type.

Because we're forced to use JSON Schema draft 4 we don't have access to the const property, but we do have access to enum, and const is just syntactic sugar for a single-element enum anyway.

[avaly]

This looks great! Again! Thanks a lot for your changes!

Just a small request: can you please add a couple of oneOf and constant properties in the schema.full unit test?

[joshuat]

@avaly I've added constant to the schema.full unit test, but when adding oneOf an error surfaced so I opened #313 to address that. That PR also adds oneOf to the schema.full unit test.