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" │
├──────────────────────────────────────────┼───────┼───────┼────────────┼────────────────────┤
│ normal-modes                             │ 3533  │ 76    │ 2          │ 0                  │