Enable gc feature by default

[tolumide-ng]

The c-api crate currently enables the gc feature on its wasmtime dependency, even though it exposes a gc feature to control this behaviour. This prevents downstream users from disabling gc, since Cargo.toml doesn't allow overriding it. However, structs like RootScope are only available when gc is enabled. Removing it from default features, without other options, would break internal implementations.

Changes

1. Enable the gc feature via the wasmtime-c-api crate by default.
2. Removes the explicitly set gc feature for the wasmtime dependency in c-api, thus enabling downstream users to specify this behaviour.

Closes #12783

[alexcrichton]

Oh for this the default feature lives here instead which includes gc, so I think it's reasonable to just from [dependencies]