improve kaptan-spec as a build library and move it to libs dir

Signed-off-by: Onur Özkan <work@onurozkan.dev>

libs/kaptan-spec/tests/ui/fail_non_string_argument.rs

@@ -0,0 +1,5 @@
+use kaptan_spec::no_spec;
+
+no_spec!(42);
+
+fn main() {}