Convert the Rollup config files to have a `.mjs` extension so that Node
can natively load them as ES modules and then load them using
`import(...)` rather than the `loadConfigFile` helper.

This avoids a deprecation warning when importing `rollup/dist/loadConfigFile`:

```
(node:21339) [DEP0148] DeprecationWarning: Use of deprecated folder
mapping "./dist/" in the "exports" field module ...
```

This also makes the way the config file is loaded more transparent, since its
just a regular dynamic import.

The downside of this change is that we don't get a `warnings` object
back to print warnings that occur during bundling.
Instead have to pass an `onwarn` handler to `rollup.{rollup, watch}`.