`get_mapped_range_mut()` is now a misnomer

[kpreid]

Followup to #9042. Since that PR, get_mapped_range_mut() does not provide any &mut [u8] but only a wgpu::WriteOnly<[u8]>. Therefore, its name (and that of BufferViewMut) is arguably incorrect.

We should consider whether we want to rename it to something else, like get_mapped_range_for_write().

The WebGPU API only has a single method named getMappedRange() corresponding to our get_mapped_range() and get_mapped_range_mut(), so there is no answer to be found there.