Faster syntax highlighting

[ces42]

Hi, I finally found the time to clean up my work on speeding up syntax highlighting.

Are the comments in autoload/vimtex/syntax/core.vim somewhat understandable? Are there other things that need changing?

There's an explicit functional change in this branch: I've added highlighting for \lVert and \rVert. There's also some new restrictions on the arguments of the vimtex#syntax#core#new_env function.