Move heavy dependencies to peerDependencies

Heavy dependencies include:
- `html-minifier-terser` (in `next` branch only)
- `imagemin-pngquant`
- `imagemin-webp` (in `next` branch only)
- `svgo`

All of these take ~5 MB each.

**Benefits:**
- Less wasted space when not using a specific feature
- Faster installation

**Drawbacks:**
- More burdensome setup/maintenance
- A few wasted MB isn't the end of the world
- Their dependencies may overlap, so the "5 MB" figure could be overstated