This PR extracts Dockerfile support into an extension and removes the
built-in Dockerfile support from Zed.

There's already an existing [Dockerfile
extension](https://github.com/d1y/dockerfile.zed) that was just missing
language server support. Language server support is being added to that
extension in https://github.com/d1y/dockerfile.zed/pull/2.

Release Notes:

- Removed built-in support for Dockerfile, in favor of making it
available as an extension. The Dockerfile extension will be suggested
for download when you open a `Dockerfile`.