Restrict v0.0.7 of the `zed_extension_api` to dev builds, for now (#12170)

This PR restricts usage of v0.0.7 of the `zed_extension_api` to dev
builds, for now.

As we're still making changes to it, we don't want to ship a version of
Zed to Preview/Stable that claims to support a yet-unreleased version of
the extension API.

Release Notes:

- N/A