Add njs as a target/profile

[0xbigshaq]

Hi Samuel/Carl, I took a small side-quest few weeks ago(for fun) and used fuzzilli. It led to writing this thingy. This is my attempt to give back :D thank you for creating this project.

This PR adds njs as a new target/profile to fuzz so everyone can enjoy hehe

Note: The logic behind the hot-patching 'technique' I implemented here is: instead of using a .patch files - we leverage the way they designed their build system to 'extend' the language(add fuzzilli() builtin, make our own shell for REPRL, etc.). I found that approach more reliable and work with more revisions.