Adds three explicit parsing strategies selectable at host build time via
the compile-time constant __NITRO_JSON_MODE__:
- legacy: strict JSON.parse only; clear error suggesting JSON5 mode
- json5 : JSON5.parse only
- auto : try JSON, fall back to JSON5 (existing behaviour and default
when the flag is undefined, so older hosts keep working)
URL/MIME hints for .json5 sources are still respected. README updated
with the modes table and a Vite wiring example.