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_missing_argument.stderr

@@ -0,0 +1,7 @@
+error: missing no-spec explanation; use no_spec!("reason");
+ --> tests/ui/fail_missing_argument.rs:3:1
+  |
+3 | no_spec!();
+  | ^^^^^^^^^^
+  |
+  = note: this error originates in the macro `no_spec` (in Nightly builds, run with -Z macro-backtrace for more info)