feat: add validator composition, pluggable messages, and opt-in strict rules (v1.3.0)

[divyanshub024]

Summary

Releases 1.3.0. The headline change is that the package no longer requires
Flutter — lib/ has no Flutter imports and the flutter: environment
constraint is dropped, so the validators now run in server-side and CLI Dart
projects. Flutter form usage is unchanged.

What's new

• Composition — generic FieldValidator<T> with compose, aggregate,
  any, conditional/when/unless, transform, and fluent chaining
  (.and(), .or(), .withMessage()).
• Messages — ValidationMessage/ValidationMessages resolve errors via a
  pluggable resolver keyed by stable IDs, with no intl dependency. An
  explicit errorMessage: still takes precedence.
• New validators — IBAN, BIC, CVC, card expiry, ISBN-10/13, fixed-length
  hashes, MIME types, data URIs, ISO-8601 dates, 24-hour times, numeric
  ranges, divisibility, text boundaries, and word counts.
• Opt-in strict modes — JWT, Base32, credit card, URL, email, FQDN, and
  UUID accept configuration; all default to 1.2 behavior.
• Docs — README rewritten; new guides under doc/ for composition,
  messages, migration, and strict validation. Examples are covered by tests.
• Example app — five interactive validation flows with widget tests.

Compatibility

No public symbol was removed: all 42 existing Validator.* factories are
retained and 20 added. Non-required validators still accept null and empty so
they compose with required. Strict validation is opt-in.

Note: Validator is now abstract final, so it can no longer be
instantiated or subclassed. It has only ever exposed static members.

Verification

dart format · dart analyze · 184 Dart tests · flutter analyze ·
5 widget tests · 96.64% line coverage · dart doc with zero warnings ·
pure-Dart smoke test · dart pub publish --dry-run with zero warnings.

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Supersedes #5, which GitHub auto-closed when its head branch was renamed.

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