Extension for "border" sampler addressing mode

[kvark]

Is your feature request related to a problem? Please describe.
Veloren uses this mode (VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_BORDER in Vk).

Describe the solution you'd like
WebGPU currently doesn't expose it, but it's supported on a lot of platforms (everything except iOS, roughly speaking), so an extension is possible.

Describe alternatives you've considered
Not exposing anything.

Additional context
https://developer.apple.com/documentation/metal/mtlsampleraddressmode/mtlsampleraddressmodeclamptobordercolor?language=objc