338: Add Extension/Limit Interface r=kvark a=cwfitzgerald
Follow up to https://github.com/gfx-rs/wgpu/pull/690.
This forwards the extension/limit access interface into wgpu-rs.
It appears that the web-sys doesn't actually implement any of the methods we need, so I had them return the defaults. `Device::limits` exists, but it returns `Object` not `GpuLimits`, so doesn't appear usable.
Co-authored-by: Connor Fitzgerald <connorwadefitzgerald@gmail.com>