Tasks that use "builtin" steps do not work after projen eject

[Chriscbr]

The current reason for this is that "builtin" steps depend on TypeScript/JavaScript files that are included in the projen distribution. Once projen is removed from the project and uninstalled, the files in node_modules/projen/... can no longer be accessed.

One solution that comes to mind is finding a way to bundle these files and add them to the "scripts" directory after projen is ejected.

Another solution could be to extract the projen task runner into its own tool/dependency.