[BREAKING] Make Shader(Module)Source::Wgsl optional

[daxpedda]

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
Discussed in Matrix.

Description
This is totally a breaking change! It also might turn out entirely unnecessary if the compile-time savings aren't worth the added complexity.

At the moment the naga wgsl-in crate feature is active even when completely unnecessary. This addresses it by gating Shader(Module)Source::Wgsl behind a new wgsl crate feature, like with ShaderSource::Glsl for example. The added wgsl crate feature is also active by default.

Code quality is definitely a bit worse with all of these #[cfg(...)] now. CI time is increased because --no-default-features has to be tested.

I also didn't know what to do about Shader(Module)Sources lifetime, at the moment the lifetime is not present when the only active variant is Naga, which I consider very user unfriendly and could only be fixed by either stop using Cow or adding a dummy lifetime to the Naga variant. This could also be solved by #2903, so maybe we should merge that first. (can't bind 'a to the 'static in Cow)

I also had to increase the MSRV to 1.60 to not unnecessarily pull in wgpu-core on WASM by using the weak dependency feature.

Testing
Added additional runs with --no-default-features in the CI.

[daxpedda]

Okay, CI just has issues installing Rust, I will try again later.

[daxpedda]

Rebased after #2872.

[daxpedda]

Rebased again.
Too bad it didn't make the cut for 0.14. @cwfitzgerald can we get this in before it gets forgotten again?

[cwfitzgerald]

Ack! So sorry about this! I didn't realize the "changes requested" text doesn't get reset when the changes get addressed.

Small comments but then g2g

[cwfitzgerald]

G2G! Please just add a changelog entry that mentions there's a new feature, but it's default.

[daxpedda]

Done.