Switch from CJS (CommonJS) to ESM (ECMAScript Modules)

[JoshuaKGoldberg]

See also yeoman/yo#787: many Yeoman have generally switched from being CJS to ESM. generator-generator is still CJS, so trying to use the latest yeoman-generator or other packages results in the dreaded:

require() of ES Module /Users/josh/repos/generator-generator/node_modules/yeoman-generator/dist/index.js from /Users/josh/repos/generator-generator/app/index.js not supported.
Instead change the require of /Users/josh/repos/generator-generator/node_modules/yeoman-generator/dist/index.js in /Users/josh/repos/generator-generator/app/index.js to a dynamic import() which is available in all CommonJS modules.

This issue tracks updating generator-generator with a "type": "module" package so that it can directly import from ESM dependencies.