Remove wgpu feature. Add wgpu_27 and wgpu_28 features.

[nicoburns]

Objective

Allow the same version of Vello to compile with multiple versions of WGPU (current v27 and v28):

• This should make it easier to upgrade to new versions of WGPU promptly without worrying about leaving users who may still need an older WGPU version behind.
• In many cases, it may also make upgrading WGPU a non-breaking change.

Prior Art

The i-slint-core crate has similar feature flags for WGPU versions (https://docs.rs/crate/i-slint-core/latest/features)

Changes made

• Replace the wgpu and naga feature flags in the vello, vello_hybrid, vello_shaders and vello_sparse_shaders crates with wgpu_27, wgpu_28 (and naga_27, naga_28) feature flags.
• Re-export e.g. wgpu_28 as wgpu in the crate roots
• The wgpu-profiler and hot-reload debug features are currently configured to only work with the latest WGPU version.

Todo

• Use crates.io version of wgpu-profiler once Update wgpu dependency to version 28 Wumpf/wgpu-profiler#103 lands