Mark prettier config file as CommonJS

I *think* this will fix a problem with the release process in which we saw an
error:

```
Error:  Invalid configuration for file "/home/runner/work/matrix-js-sdk/matrix-js-sdk/package.json":
Error:  module is not defined in ES module scope
Error:  This file is being treated as an ES module because it has a '.js' file extension and '/home/runner/work/matrix-js-sdk/matrix-js-sdk/package.json' contains "type": "module". To treat it as a CommonJS script, rename it to use the '.cjs' file extension.
```