add readonly option

[albertnahas]

feat: add readonly option to prepend readonly to fields

This feature adds a new readonly option to json-schema-to-typescript, enabling users to prepend readonly to all fields in generated TypeScript interfaces.

Changes:

• Added logic to prepend readonly to fields in generator.ts.
• Updated the Options interface to include the new readonly flag.
• Updated README to document the readonly option.

Testing:

• Added test cases to validate the readonly option.
• Ensured all existing tests pass to maintain backward compatibility.

Closes #627