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_empty_explanation.rs

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

libs/kaptan-spec/tests/ui/fail_empty_explanation.stderr

@@ -0,0 +1,7 @@
+error[E0080]: evaluation panicked: no-spec explanation must not be empty; use no_spec!("reason");
+ --> tests/ui/fail_empty_explanation.rs:3:1
+  |
+3 | no_spec!("   ");
+  | ^^^^^^^^^^^^^^^ evaluation of `_` failed here
+  |
+  = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `no_spec` (in Nightly builds, run with -Z macro-backtrace for more info)

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

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

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)

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

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

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 {
+  |              ^^^^^^^^^^^^^^^^^^

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

@@ -0,0 +1,5 @@
+use kaptan_spec::no_spec;
+
+no_spec!("This fixture has no governed runtime behavior.");
+
+fn main() {}