This adds a prompt to the create-next-app CLI to ask if the user wants to use
TypeScript or JavaScript.

It also adds tests for the different templates (JS, TS, and experimentalApp),
and some helpers to check whether an emitted project contains the correct files:

- projectFilesShouldExist({ projectName, cwd, files })
- projectFilesShouldNotExist({ projectName, cwd, files })

And higher-level helpers for checking that an emitted project contains the
correct files per a template specification in
test/integration/create-next-app/lib/projectFiles:

- shouldBeJavaScriptProject({ projectName, cwd })
- shouldBeTypeScriptProject({ projectName, cwd })
- shouldBeAppProject({ projectName, cwd })

…est spec (#3)

* chore(create-next-app): restructure templates

* chore: add project template specification

* chore: add JS `app` template

* chore: lint