feat: add failOnConflicts and conflictLabel options for better conflict handling

[kapral18]

Summary

Adds two new configuration options to improve conflict visibility when using commitConflicts: true:

• failOnConflicts (default: true): Controls exit code behavior when PRs are created with conflicts
• conflictLabel (default: "merge-conflict"): Customizable label automatically added to PRs with conflicts

Motivation

When commitConflicts: true, backport creates PRs even with merge conflicts. However, the process exits with code 0, causing GitHub Actions to show as SUCCESS (green). This makes it easy to overlook PRs that need manual conflict resolution.

Partially addresses: #447 (comment)

Changes

• Added failOnConflicts option that causes the process to exit with code 1 when conflicts are detected (only in non-interactive mode with commitConflicts: true)
• Added conflictLabel option to customize the label added to conflicted PRs
• Modified backport response to include hasConflicts flag for each result
• Updated logic to automatically label PRs that have merge conflicts
• Added comprehensive tests for conflict scenarios including new E2E test helpers
• Updated documentation for both CLI and config file options

Behavior

With commitConflicts: true AND failOnConflicts: true (default):

• Conflicts encountered → Conflicts committed → PR created with conflict markers → Exit code 1 ✅
• Result: GitHub Action shows as FAILED (red), making conflicts visible

With commitConflicts: true AND failOnConflicts: false:

• Conflicts encountered → Conflicts committed → PR created with conflict markers → Exit code 0 ⚠️
• Result: GitHub Action shows as SUCCESS (green), conflicts may be overlooked

Test plan

• Unit tests added for new CLI options
• Unit tests for conflict detection and labeling
• E2E mutation tests for conflict scenarios with custom labels
• E2E tests verified against https://github.com/kapral18/backport-conflict-test
• Documentation updated