Enable JSON Schema multi file output

[kmturley]

This PR adds support for json schema multi file output.

Without multi-file-output option:

With multi-file-output option:

Tested with this repo:
https://github.com/kmturley/quicktype-typescript-input-demo

Replacing the packages/quicktype-core/dist/language/JSONSchema.js file with the compiled file from this PR

[schani]

Thanks for this — a multi-file JSON Schema output (one file per definition, e.g. for schema registries or per-type $ids) is a feature we want, and the overall shape here is right: single-file output stays byte-for-byte unchanged, and you correctly reused the existing startFile/finishFile/initializeEmitContextForFilename machinery other multi-file renderers already use.

A few things need to be resolved before this can merge, though:

• Blocker: cross-file $refs dangle. makeRef (JSONSchemaRenderer.ts, untouched by this PR) always emits a same-document pointer, { $ref: "#/definitions/${name}" }. Once each definition moves to its own file, any reference to another definition points at a definitions block that no longer exists in that file — the output isn't valid/resolvable JSON Schema in multi-file mode. makeRef needs to be ref-aware, e.g. emitting Other.schema#/definitions/Other for cross-file references when multiFileOutput is on.
• Every per-definition file embeds the $schema header and the top-level root type/ref. outputFile (the new private helper in JSONSchemaRenderer.ts) is called both for the root/stdout case and for each object/union/enum, and unconditionally prepends $schema plus topLevelType. That's wrong for every file except the root — per-definition files shouldn't restate the document's root type.
• No test coverage. Per repo convention this needs end-to-end fixture coverage, but there's also no harness support yet to validate multi-file output at all (every emitted file parses, every cross-file $ref resolves) — needs adding to the JSON Schema fixture path in test/fixtures.ts.
• Merge conflicts with current master in both changed files (JSONSchemaRenderer.ts, language.ts) — ESM .js import suffixes, biome formatting (object literal vs. Object.assign), and getOptions(): Record<string, never> typing. Mechanical, but need a rebase.
• Minor: outputFile("stdout", ...) passes a filename that's never used in single-file mode (since startFile/endFile short-circuit when multiFileOutput === false); and the new startFile doc comment (copied from GolangRenderer) says it lowercases the filename, but this implementation doesn't.

Given the size of the ref-rewriting fix (item 1) plus the missing test infrastructure, this needs another pass. @kmturley, want to pick it back up? If not, we'd like to reimplement using this as the starting point, with credit for the original approach.