Add `ExternalImageSource::VideoFrame`

[jprochazk]

WebCodecs API introduces VideoFrame (https://w3c.github.io/webcodecs/#videoframe), which is a valid ExternalImageSource in both WebGL (WebGLRenderingContext.texImage2D) and WebGPU (GPUQueue.copyExternalImageToTexture) contexts.

[cwfitzgerald]

We'd love a PR for this, should just need to be added to the enum, like #5668 did