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.stderr

@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+ --> tests/ui/fail_non_string_argument.rs:3:10
+  |
+3 | no_spec!(42);
+  |          ^^ expected `&str`, found integer
+
+error[E0308]: mismatched types
+ --> tests/ui/fail_non_string_argument.rs:3:10
+  |
+3 | no_spec!(42);
+  | ---------^^-
+  | |        |
+  | |        expected `&str`, found integer
+  | arguments to this function are incorrect
+  |
+note: function defined here
+ --> src/lib.rs
+  |
+  | pub const fn has_non_whitespace(value: &str) -> bool {
+  |              ^^^^^^^^^^^^^^^^^^