make spec infra more ergonomic

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

kaptan-spec/tests/ui/fail_non_string_argument.stderr

@@ -1,7 +1,7 @@
-error: invalid spec attribute; use #[spec("file.spec.md")]
+error: invalid no-spec invocation; use no_spec!("reason");
  --> tests/ui/fail_non_string_argument.rs:3:1
   |
-3 | #[spec(42)]
-  | ^^^^^^^^^^^
+3 | no_spec!(42);
+  | ^^^^^^^^^^^^
   |
-  = note: this error originates in the attribute macro `spec` (in Nightly builds, run with -Z macro-backtrace for more info)
+  = note: this error originates in the macro `no_spec` (in Nightly builds, run with -Z macro-backtrace for more info)