This PR adds a GitHub Action for publishing the extension CLI.

When the `extension-cli` tag is pushed, this Action will run, build the
`zed-extension` binary, and upload it to DigitalOcean for consumption.

This will allow us to consume the pre-built binary in the CI for the
extensions repo.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>