Add support for plain JavaScript (No TypeScript)

[akd-io]

To make it as easy as possible I've been toying with the idea of renaming and transpiling the TypeScript files to JavaScript to strip them of their types. Other than that:

• create-next-app should be called without the --typescript flag.
• All ".ts" and ".tsx" extension references in generated code and config files should be updated to support both JavaScript and TypeScript. (This will make it easier for users to add TS support later)