changed websys features in core and hal to align with used features

[Seamooo]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
addresses #2894

Description
if using wgpu-core or wgpu-hal as a dependency with target wasm32-unknown-unknown without specifying the web-sys feature: OffscreenCanvas in the dependencies, both wgpu-core and wgpu-hal would fail to compile (wgpu-hal only when using the gles feature which is effectively implied by the target). The trivial fix to this is adding OffscreenCanvas to the web-sys features in wgpu-hal, as wgpu-core also depends on it whenever needing to use web-sys, however, for semantics sake, the feature is in use in wgpu-core as well, so has been added, however behind a more restrictive target as that's the only time web-sys is in use.

Testing
inside wgpu/wgpu-core:

• cargo build --target wasm32-unknown-unknown
• cargo build --target wasm32-unknown-emscripten

inside wgpu/wgpu-hal

• cargo build --target wasm32-unknown-unknown --features gles
• cargo build --target wasm32-unknown-emscripten --features emscripten

all builds succeed, whereas previously the wasm32-unknown-unknown target builds would fail

[cwfitzgerald]

Thanks!