Add fuzz targets for ES6 modules, JSON, RegExp, and bytecode (#512)

Add fuzz targets for ES6 modules, JSON, RegExp, and bytecode (#512)

* Add fuzz targets for ES6 modules, JSON, RegExp, and bytecode

Adds 4 new fuzzers targeting high-complexity, low-coverage functions:

- fuzz_module_export: Tests ES6 module export/import parsing (complexity 6727)

- fuzz_json: Tests JSON stringify/parse (complexity ~5000)

- fuzz_regexp_compile: Tests RegExp compilation (complexity 5528)

- fuzz_bytecode: Tests bytecode execution (complexity 5383)

Identified by Fuzz Introspector as having 0% runtime coverage.

Build integration for fuzz/Makefile and build.sh included.

* Convert fuzz targets from C++ to C and use standard C headers