Make `create-entry` available in themes

### Description

There is currently an expectation that `create-entry` will be run within a plugin, as the output uses file paths constructed with `plugin_url()` ([example](https://github.com/alleyinteractive/alley-scripts/blob/main/packages/create-entry/templates/entry/index.php.mustache#L26)). Should we consider modifying this to allow developers to run the command within themes? It would need to be a prompt in order to modify the current behavior. Alternately, we could choose a way of getting the URL to the file without relying on plugin/theme specific function calls.