Register URL handler for code scheme

code-server should be able to handle the `code` scheme, so URLs like `vscode:extension/kayteh.plastic-slate` such as used to install themes via a **Open in VSCode** button e.g. on the [Plastic Slate theme](https://vscodethemes.com/e/kayteh.plastic-slate) page on [VSCodeThemes](https://vscodethemes.com).

**Edit**: not sure if we can handle `code` because the protocol must start with `web+`. Handling `web+code-server` might make sense though.