Add "type":"module" to published root package.json file.

The root package.json file was neglected in #8396, since it isn't
generated in the same way as the other (nested) package.json files.

This commit puts `"type": "module"` in the _published_
`@apollo/client/package.json` file, which is generated at build time as
the file `dist/package.json`. The root `package.json` file that's
checked into the repository does _not_ contain `"type": "module"` (even
though putting it there would also put it in `dist/package.json`).
Attempting to enable ES module syntax at that level interfered pretty
severely with tools like ts-node, which we use to run various
config/*.ts scripts. Rewriting those scripts is a job for another time.

I'm happy to revisit this as the ecosystem progresses.

config/prepareDist.js

@@ -23,6 +23,12 @@ const distRoot = `${__dirname}/../dist`;
 const packageJson = require('../package.json');
 const entryPoints = require('./entryPoints.js');
 
+// Enable default interpretation of .js files as ECMAScript modules. We don't
+// put this in the source ../package.json file because it interferes with tools
+// like ts-node, which we use to run various ../config/*.ts scripts.
+// TODO(benjamn) Fully diagnose that interference.
+packageJson.type = 'module';
+
 // The root package.json is marked as private to prevent publishing
 // from happening in the root of the project. This sets the package back to
 // public so it can be published from the "dist" directory.