theme_importer: Add ability to print theme JSON schema (#7129)

This PR adds a quick subcommand to the `theme_importer` to facilitate
printing out the JSON schema for a theme.

Note that you do need to pass a `<PATH>` to the subcommand still, even
though it will be ignored. I'll rework the CLI to this at some point.

The JSON schema for the current version of the theme can also be found
at
[`https://zed.dev/schema/themes/v0.1.0.json`](https://zed.dev/schema/themes/v0.1.0.json).

Release Notes:

- N/A