Convert to TypeScript

[brent-phet]

This is a placeholder for a future conversion to TypeScript, as time and/or urgency permits. The current TypeScript conversion checklist is located here: https://github.com/phetsims/phet-info/blob/main/typescript-conversion/conversion-checklist.md

As of 8/12/25, the status of this sims' JS to TS is:

│ (index)                                  │ JS    │ TS    │ % Complete │ "@ts-expect-error" │
├──────────────────────────────────────────┼───────┼───────┼────────────┼────────────────────┤
│ function-builder-basics                  │ 665   │ 167   │ 20         │ 0                  │

[pixelzoom]

Most of the code for this sim comes from function-builder repo, but it does have it's own code for the Mystery screen.

phetsims/function-builder#158 provides more information and notes.