Some improvements to JSONSchemaType for records

[erikbrinkman]

What issue does this pull request resolve?

fixes #1491

What changes did you make?

For JSONSchemaType: additional properties doesn't required type: "string" since it's
implied. required is no long required for objects without any required
members.

Is there anything that requires more attention while reviewing?

1. While I think that ultimately this makes the type support better for records, It does make the type definition itself a bit messier, so I'm open to the perspective that that messiness isn't worth the improvement
2. I chose to keep required required for partial types. I think that makes sense, but it's worth calling out.
3. The tests are only type tests, I can add schema validation tests too if you think it's necessary

[epoberezkin]

I am wondering - why "required" is required in partial schemas? I am probably missing something, as I write the example, it does not seem required (even when it has to be). This example type checks:

type MyUnion = {tag: "a"; a: number} | {tag: "b"; b: number}

const myUnionSchema: JSONSchemaType<MyUnion> = {
  type: "object",
  required: ["tag"],
  oneOf: [
    {
      properties: {
        tag: {const: "a"},
        a: {type: "number"},
      },
    },
    {
      properties: {
        tag: {const: "b"},
        b: {type: "number"},
      },
    },
  ],
};

but it probably it should have required: ["a"] (or "b") in subschemas...

Either way, it's unrelated issue - I am going to merge it as is - thank you very much!

[erikbrinkman]

I think it doesn't matter because because partial schemas are just that, partial:

ajv/lib/types/json-schema.ts

Line 4 in c9e0676

export type PartialSchema<T> = Partial<JSONSchemaType<T, true>>