feat: add detect-invisible-unicode action

[KlausNie]

Examples:

• https://github.com/QuickBirdEng/test-repo/pull/9
• https://github.com/QuickBirdEng/test-repo/pull/7

Summary

• Adds detect-invisible-unicode composite action to scan source files for invisible Unicode characters used in supply chain attacks
• Detects GlassWorm vectors (Variation Selectors U+FE00–U+FE0F, U+E0100–U+E01EF) and Trojan Source bidirectional control characters
• Also covers zero-width chars, hidden BOM, Tags block, and Private Use Area characters
• Skips binary files automatically; emits GitHub Actions annotations with file and line number per finding

Test plan

• Call the action on a repo with a file containing a variation selector character — should fail with ::error annotation
• Call the action on a clean repo — should exit 0 with "No invisible Unicode characters detected"
• Verify exclude-dirs and exclude-patterns inputs correctly skip matching paths
• Verify fail-on-found: false reports findings without failing the job