Add string escape high-lighting

queries/highlights.scm

@@ -33,6 +33,7 @@
 (nil_literal) @constant.builtin
 (integer_literal) @number
 (string_literal) @string
+(escape_sequence) @string.escape
 ; }}}
 
 ; Operators {{{

test/highlight/literals.tig

@@ -2,5 +2,7 @@ nil
 /* <- constant.builtin */
 42
 /* <- number */
-"Hello World!"
+"Hello World!\n"
-/* <- string */
+/* <- string
+             ^ string.escape
+*/