refactor: types ARButton and VRButton

[DylanTackoor]

What

• ARButton.js -> ARButton.ts
• VRButton.js -> VRButton.ts

Why

Types XRButtons. Didn't see an issue tracking these in any of the typing milestones so no worries if it's too early to merge this. Just wanted to fix something inbetween working on typing the loaders as they're taking a while.

Checklist

• [x] Ready to be merged

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[CodyJasonBennett]

Continuing in #241. three.js has broken a lot of stuff around WebXR and its renderers since.