wasmtime(gc): Add the `wasmtime::NoExtern` value type

[fitzgen]

[github-actions[bot]]

Subscribe to Label Action

cc @fitzgen

This issue or pull request has been labeled: "wasmtime:api", "wasmtime:ref-types"

Thus the following users have been cc'd because of the following labels:

• fitzgen: wasmtime:ref-types

To subscribe or unsubscribe from this label, edit the .github/subscribe-to-label.json configuration file.

Learn more.