Create a subcommand in `extension` for "activating" built-in extensions

Using a new extension currently requires that a developer knows how to write the `using extension [ext-name];` statement in their schema _and_ that they need to now generate and apply a migration before it's ready to use. We should add a CLI command that is sugar for this.

## Notes

- Naming ideas: `use`, `using`, `install`, `add`
- Maybe add a `--no-migration` switch
- Detect dev-mode and skip migration
- Add a new `ext.gel` file and add extensions _there_ instead of `default.gel`?