Ensure a verbatim tag uri adheres to the spec

* A global tag uri prefix must with `tag:` if declared in verbatim.
* Add more tests.

Author: kekavc24

pkgs/yaml/lib/src/scanner.dart

@@ -1068,16 +1068,14 @@ class Scanner {
     } else {
       tagUri = _scanTagUri(); // !<foo:uri>
 
-      // Expect !<foo:uri> to be !<tag:yaml.org,2002:*>
-      switch (tagUri.replaceFirst('tag:yaml.org,2002:', '')) {
-        case 'map' || 'seq' || 'str' || 'null' || 'bool' || 'int' || 'float':
-          break;
-
-        default:
-          throw YamlException(
-            'Invalid tag uri used as a verbatim tag',
-            _scanner.spanFrom(start),
-          );
+      /// Expect !<foo:uri> to be !<tag:*> (a global tag)
+      ///
+      /// See: https://yaml.org/spec/1.2.2/#3212-tags
+      if (!tagUri.startsWith('tag:') || tagUri.substring(4).isEmpty) {
+        throw YamlException(
+          'Invalid tag uri used as a verbatim tag',
+          _scanner.spanFrom(start),
+        );
       }
     }
 

pkgs/yaml/test/yaml_test.dart

@@ -1140,6 +1140,8 @@ void main() {
     test('[Example 6.25]', () {
       expectYamlFails('- !<!> foo');
       expectYamlFails('- !<\$:?> foo');
+      expectYamlFails('- !<tag:>'); // Incomplete verbatim tag uri
+      expectYamlFails('- !<improvised:tag>');
     });
 
     test('[Example 6.26]', () {