Feat conflict detection

[solssak]

🎯 Changes

This PR implements hotkey conflict detection with configurable behavior strategies, addressing the "Warn/error on conflicting shortcuts (TBD)" roadmap item from the README.

Motivation:
When multiple components or modules register the same hotkey, developers need control over how these conflicts are handled. Without this feature, duplicate registrations silently coexist, which can lead to unexpected behavior and difficult debugging.

Implementation:

• Added ConflictBehavior type with 4 strategies: 'warn', 'error', 'replace', 'allow'
• Added conflictBehavior option to HotkeyOptions (defaults to 'warn' for safety)
• Implemented #findConflictingRegistration() and #handleConflict() private methods
• Added 5 comprehensive test cases covering all scenarios
• Fixed ESLint no-unnecessary-condition warning
  Usage:

// Default: warn but allow both registrations
manager.register('Mod+S', handler1)
manager.register('Mod+S', handler2) // ⚠️ console.warn
// Strict mode: prevent conflicts
manager.register('Mod+S', handler, { conflictBehavior: 'error' }) // throws
// Override mode: replace existing
manager.register('Mod+S', newHandler, { conflictBehavior: 'replace' })
// Silent mode: intentional duplicates
manager.register('Mod+S', handler, { conflictBehavior: 'allow' })
Note: Conflicts are only detected when both the hotkey AND target match. Different targets (e.g., document vs a specific element) don't conflict.

✅ Checklist

• I have followed the steps in the Contributing guide (https://github.com/TanStack/keys/blob/main/CONTRIBUTING.md).
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset (https://github.com/changesets/changesets/blob/main/docs/adding-a-changeset.md).
• This change is docs/CI/dev-only (no release).