Add keycode fes warning

[a-y-a-n-das]

Resolves #7876

Changes:

• Added a one-time Friendly Error System (FES) warning for keyCode usage in src/core/main.js.
  • The warning informs users that keyCode is deprecated and suggests using keyIsDown() instead.
• Added a manual test files: test/manual-test-examples/events/keyCode-warning.html and test/manual-test-examples/events/keyCode-warning-sketch.js
  • Verifies that the warning appears once and key behavior remains unchanged.

Screenshots of the change:

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

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Thank You!

[a-y-a-n-das]

Hi!
I realized I should’ve claimed this issue first (as per the contributing guidelines), but since it wasn’t assigned to anyone and was still open, I went ahead and fixed it.
Hope that’s okay — please review when you get a chance. I can make any necessary adjustments if needed. 😊