Support for `const` definitions

[tshelburne]

Currently we have to use the following schema to define constants:

value:
  type: string
  enum: [some-constant]

Which results in:

type value = 'some-constant'

JSON schema supports defining with the const property, and it looks like OAI has adopted it as well? The definition would look like this:

value:
  type: string
  const: some-constant

And would result in the same Typescript as above.

[sadfsdfdsa]

Working on it.

UPD: #831

[drwpow]

This was added in v5 a while back, and support has improved for the upcoming v6 version.