Highlighting strings

Hi thanks for this, it worked straight away for me.
My suggestions:

change line 2 to:
<UserLang name="G Code" ext="nc g gcode TAP" udlVersion="2.1">
so that files with the ".gcode" extension get the syntax highlighting too.
I'll see if I can make a pull request for this.

Also, is there a way to colour comments and strings with a single colour? Anything with a proceeding semicolon should be highlighted perhaps grey.

Also, after commands like M117 the argument is a string, so these strings should be formatted with another single colour, perhaps green.

I added the gcode extension, and support for semicolon comments, but I could not push or create a pull request as I do not have permission.