Fix FES stack frame access for internal errors

[davepagurek]

Resolves #8174

Changes:

• Changes the index offset we use when identifying the function the user called that triggered an internal error

I was trying to figure out why this was -2 originally. I think maybe at one point we were wrapping internal functions differently? But it looks like we no longer do, and this can match the -1 that we have in 1.x.

Test sketch to see the fix working: https://editor.p5js.org/davepagurek/sketches/eXp2Vc7Xo

PR Checklist

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