Add NASM (Netwide Assembler) language definition.

This is an update of https://github.com/LeaVerou/prism/pull/365 - see previous PR for discussion.

I have tested this and it works for my files.

Credit to @zeitgeist87 for his help merging the regexes together.
This doesn't highlight ymm or zmm (AVX2 and AVX-512 registers).