The "New" command should have better UX

[joakimbeng]

The generator plugins should be reworked into "template plugins", where each template plugin can provide multiple templates and each template have a file extension. The template should automatically be picked based on the --extension option, and if multiple templates match (or if the --extension or --template options are not specified) the user should be prompted for which template (from which plugin) to use, and Emigrate should come with some default templates for CommonJS and ESM.