Prefpicker

- Added writable-streams variant

*This release includes breaking changes!*
- template format changes
- API changes (pathlib is used where possible)
- Added PrefPicker.lookup_template()

- Enable ReadableStreamDefaultController interface
- Remove obsolete layout prefs
- Enable gfx.offscreencanvas.enabled

- Fission is now default
- Disable WebGL warnings in stderr

- Re-enable large array buffers
- Enable WebLocks

- Set `network.proxy.allow_bypass=false`
- Don't force `dom.vr.enable=false` by default

- Remove WR variant (now default)