Add TypeScript types for core

**Problem**
When working with parsers, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

No files, tests, or entry points are named. First reconcile the title’s TypeScript type request with the body’s parser error-message request, then locate the core parser code and existing type definitions. Done should be an agreed, specific change with tests covering the requested behavior.

Written by the indexing model from the issue text.