fix(gen): isolate generator to `commonjs` (#8109)
### Description
As described by #5657, the default `simple-*` templates creates a
template that is broken for `"type": "module"` packages.
When digging into this issue I found out that a simple `package.json`
with `"type": "commonjs"` solves the problems by telling the runtime to
treat the file diferently from the parent package.
### Testing Instructions
- Initialize a new monorepo with `"type": "module"` in the
`package.json`
- on this branch it should now be possible to generate the sample
generator (either `js` or `ts`) and not have any errors about the module
type.
- it should also be possible to run the generator and it should create
the file in the project root normally.
resolves #5657
