fix(ts): added propertyNames support to ts

[carloscasellas]

Description

Adds support for JSON Schema's propertyNames keyword when it restricts an object's keys to a finite set of strings (via enum or const). Previously this constraint was silently ignored.

• For TypeScript, a map whose keys are restricted this way is now generated as Partial<Record<"key1" | "key2", V>> instead of an unconstrained index signature.
• For JavaScript, the runtime converter now validates incoming keys against the allowed set and rejects unknown ones.
• If some of the restricted keys are also required, they're expanded into explicit (non-optional) class properties rather than left as an unconstrained map.
• Flow, Python, Go, and other languages are intentionally unaffected — they keep their existing (unconstrained) map/dict output, since they either lack the type-system feature to express it or it was out of scope for this change.

Related Issue

Fixes #1959

Motivation and Context

propertyNames restricting keys to an enum was ignored entirely, so { [key: string]: T } was generated with nothing indicating which keys are actually allowed — losing type safety and letting invalid keys through undetected at runtime in JS/TS.

Previous Behaviour / Output

Given a schema like:

{
  "languages": {
    "type": "object",
    "propertyNames": { "type": "string", "enum": ["de", "en"] },
    "additionalProperties": { "type": "object", "properties": { "name": { "type": "string" } } }
  }
}

TypeScript generated:

languages: { [key: string]: Language };

with no indication that only de and en are valid keys, and the JS runtime converter accepted any key.

New Behaviour / Output

TypeScript now generates:

languages: Partial<Record<"de" | "en", Language>>;

and the JavaScript runtime converter rejects any key outside ["de", "en"] with an invalidValue error.

If a subset of the allowed keys is also required, they become explicit properties, e.g.:

de?: Language;
en:  Language;

How Has This Been Tested?

• New fixture test test/inputs/schema/property-names.schema, with a positive sample (property-names.1.json, property-names.2.json) and a negative sample (property-names.1.fail.property-names.json) that must be rejected at runtime by languages declaring the new property-names feature (TypeScript, JavaScript, Flow — enabled in test/languages.ts).
• New unit test suite test/unit/property-names-schema.test.ts (12 tests) covering:
  • enum/const constraints producing Partial<Record<...>>
  • keys staying optional (not a bare Record<...>)
  • required keys expanding into explicit properties
  • the contradiction case (required + additionalProperties: false) still erroring
  • non-finite constraints (pattern, mixed-type enum, $ref) correctly leaving the map unconstrained
  • key escaping for non-identifier strings
  • Flow, Python, and Go outputs remaining unchanged
• Ran tsc --noEmit on quicktype-core — clean.
• npm run test:unit — all passing.