feat: Validator.compose and stricter URL, date, and int checks

[divyanshub024]

Summary

• Add Validator.compose so Flutter forms can combine required with other rules without ?? chaining; drop the circular barrel import in form_validator.dart.
• Treat empty-host URLs (https://, http://.), calendar overflow (2023-13-01, 2023-02-29), and hex integer literals (0x10) as invalid.
• Make the package pure Dart (no Flutter SDK constraint), switch CI to dart-lang/setup-dart, and cache hot-path regexes.

Test plan

• dart analyze and dart test pass
• Validator.compose([required(), email()]) rejects empty/null/whitespace and invalid emails, and accepts a real email
• https://, http://., and http://foo fail isURL; http://localhost and https://example.com still pass
• 2023-13-01 and 2023-02-29 fail isDate; 2024-02-29 and ISO timestamps still pass
• 0x10 / 0X10 fail isInt; '01' and '123 ' still pass
• Example form email field uses compose so an empty submit is rejected
• pubspec.yaml has no flutter: environment key