slash_commands_example: Improve setup instructions in README (#26217)

This PR improves the setup instructions for the slash-commands-example
extension by:

1. Replacing the `sed` command with a more reliable approach that
completely replaces the Cargo.toml file.

2. Explicitly showing how to create a standalone extension with a
properly configured Cargo.toml file that:
   - Uses `edition = "2021"` instead of `edition.workspace = true`
   - Doesn't include `publish.workspace = true`
   - Doesn't include the `[lints]` section

This change addresses an issue where the extension wouldn't work when
copied as a standalone project due to workspace references that are only
valid when the extension is built as part of the main Zed repository.

The updated instructions provide a clear, reliable path for developers
to create their own Zed extensions based on the slash commands example.

Release Notes:

- N/A