fix(gen): isolate generator to commonjs (vercel#8109)

### Description

As described by vercel#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 vercel#5657

packages/turbo-gen/src/templates/simple-js/package.json

@@ -0,0 +1,3 @@
+{
+ "type": "commonjs"
+}

packages/turbo-gen/src/templates/simple-ts/package.json

@@ -0,0 +1,3 @@
+{
+ "type": "commonjs"
+}