Investigate replacing typescript-json-schema with a tree-sitter-based TypeScript front end

[schani]

Split out from discussion on #3037.

quicktype-typescript-input currently converts .ts sources to JSON Schema via the third-party typescript-json-schema package. Two recurring problems with this approach:

• It pins us to a specific TypeScript compiler version, and upgrades have repeatedly required forking/patching (e.g. bigint handling, JSDoc @type braces, enum member names).
• The conversion is increasingly patched with post-processing passes on top of the generated schema, which is becoming hard to maintain.

We should investigate whether a tree-sitter-based parser for the TypeScript input could give us a lighter, version-independent front end that produces quicktype-core input directly, and what functionality (type resolution, Required<>/utility types, cross-file refs) we would lose or have to reimplement.

This is an investigation/design task, not a committed rewrite.

[schani]

Closing as not planned after investigating the tradeoff.

The key distinction is that the current front end does not merely parse TypeScript syntax. Through the TypeScript TypeChecker, typescript-json-schema sees resolved semantic types: imports and symbols have been resolved, generics instantiated, declarations merged, mapped/indexed/utility types evaluated, and computed enum or inferred expression types determined.

Tree-sitter would give us only the syntax tree. It can tell us that a declaration contains Required<Pick<Person, "name">>, T[K], or ReturnType<typeof fn>, but not the resulting object shape. Preserving everything the current input handles correctly would therefore require quicktype to implement a substantial TypeScript name resolver and type evaluator. That would amount to maintaining a partial TypeScript type checker, which is much more complexity and compatibility risk than the compiler-version dependency this was intended to remove.

JSON Schema is still the appropriate intermediate representation. If we revisit this area, a more realistic direction would be to keep TypeScript's TypeChecker while replacing typescript-json-schema with a quicktype-owned TypeChecker-to-JSON-Schema converter. That could remove the private generator patches and accumulated schema post-processing without reimplementing TypeScript semantics, but it would not eliminate the TypeScript compiler dependency and should be scoped separately.