feat: updated bool parsing and added tests

tests/test_errors.rs

@@ -3,11 +3,13 @@ mod test_errors {
     use envkit::error::EnvError;
 
     #[test]
+    /// Tests for the `missing` function.
     fn test_missing_var() {
         assert_eq!(EnvError::missing("missing test"), EnvError::MissingVar("missing test".to_string()));
     }
 
     #[test]
+    /// Tests for the `invalid` function.
     fn test_invalid() {
         assert_eq!(EnvError::invalid("key", "value", "message"), EnvError::Invalid {
             key: "key".to_string(),