Adds lookbehind support

[arcanis]

This PR attempts to fix an error that occurs when using lookbehind expressions within tokens:

Error: Unable to use "first char" lexer optimizations:

	Failed parsing: < /(?<=[:=<>])[a-zA-Z_]\w*(?=[:=<>])/ >
	Using the @chevrotain/regexp-to-ast library
	Please open an issue at: https://github.com/chevrotain/chevrotain/issues

Ref: bd82/regexp-to-ast#84
Ref: bd82/regexp-to-ast#129

[bd82]

looks like the CI build is broken, I'll try to fix