lua.lang: improve escape regex
lua.lang: improve escape regex

List of valid escape strings taken from
http://www.lua.org/work/doc/manual.html (3.1 - Lexical Conventions).

https://bugzilla.gnome.org/show_bug.cgi?id=738971

cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview