webidl: webgpu: make the `size` argument to `copyBufferToBuffer` optional

[andyleiserson]

This matches the latest spec.

There is also a 3-argument overload of copyBufferToBuffer, which I have not added, as it poses some additional challenges, and I do not immediately need it.

See https://www.w3.org/TR/webgpu/#commands-buffer-copies for context for the following explanation.

The logic in wasm-bindgen that generates unique names for the different overloads does not understand that the 3-argument overload is omitting the 2nd and 4th arguments to the 5-argument overload. So it generates names based on every argument starting with the 2nd, which (1) changes existing names for variants of the 5-argument overload, (2) includes with_gpu_buffer for the 3rd argument in the names even though that has no actual disambiguation purpose.

[andyleiserson]

@daxpedda apologies for the ping, do you have a minute to review this? Excluding the generated code, it's a very small change, and having it upstream helps us to avoid tracking local changes to our vendored copy. Thanks!