Add support for _ in hex numbers

Fixes pangloss#1232

I am not going for a super accurate fix, just enough to not break syntax
highlighting in a valid use case

Author: amadeus

syntax/javascript.vim

@@ -54,7 +54,7 @@ syntax match   jsModuleComma        contained /,/ skipwhite skipempty nextgroup=
 syntax region  jsString           start=+\z(["']\)+  skip=+\\\%(\z1\|$\)+  end=+\z1+ end=+$+  contains=jsSpecial extend
 syntax region  jsTemplateString   start=+`+  skip=+\\`+  end=+`+     contains=jsTemplateExpression,jsSpecial extend
 syntax match   jsTaggedTemplate   /\<\K\k*\ze`/ nextgroup=jsTemplateString
-syntax match   jsNumber           /\c\<\%(\d\+\%(e[+-]\=\d\+\)\=\|0b[01]\+\|0o\o\+\|0x\x\+\)\>/
+syntax match   jsNumber           /\c\<\%(\d\+\%(e[+-]\=\d\+\)\=\|0b[01]\+\|0o\o\+\|0x\%(\x\|_\)\+\)\>/
 syntax keyword jsNumber           Infinity
 syntax match   jsFloat            /\c\<\%(\d\+\.\d\+\|\d\+\.\|\.\d\+\)\%(e[+-]\=\d\+\)\=\>/