Add literals highlighting

queries/highlights.scm

@@ -1,3 +1,9 @@
 (comment) @comment
 
+; Literals {{{
+(nil_literal) @constant.builtin
+(integer_literal) @number
+(string_literal) @string
+; }}}
+
 ; vim: sw=2 foldmethod=marker

test/highlight/literals.tig

@@ -0,0 +1,6 @@
+nil
+/* <- constant.builtin */
+42
+/* <- number */
+"Hello World!"
+/* <- string */