make spec parser stricter

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

kaptan-spec/tests/ui/fail_legacy_named_argument.stderr

@@ -0,0 +1,7 @@
+error: invalid spec attribute; use #[spec("file.spec.md")]
+ --> tests/ui/fail_legacy_named_argument.rs:3:1
+  |
+3 | #[spec(path = "flight_core_contract.spec.md")]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `spec` (in Nightly builds, run with -Z macro-backtrace for more info)