Rename `.prettierrc` to `.prettierrc.json`

[coliff]

This means it can be formatted with Prettier and the file will have syntax highlighting and JSON linting in IDEs and on GitHub Web UI.

PR Checklist

Before creating new PR, please take a look at checklist below to make sure that you've done everything that needs to be done before we can merge it.

• [x] read and followed the CONTRIBUTING.md guide.
• [ ] built and tested the changes locally.
• [ ] added/updated tests.
• [ ] added/updated API documentation.
• [ ] added/updated demos.