This PR includes a "soft preload" feature for icons, where icons will be
loaded when the canvas first mounts. The component will not wait for
icons to finish loading before showing the editor, but this should help
with "pop in" on menu icons.
### Change Type
- [x] `sdk` — Changes the tldraw SDK
- [x] `improvement` — Improving existing features
### Test Plan
1. Load the component
2. After load, open a menu for the first time
3. The icons should immediately be visible
### Release Notes
- Improve icon preloading
1450454873