swizzle --patch

## 💥 Proposal

We can make swizzle something similar to [`yarn patch`](https://yarnpkg.com/cli/patch): it lets you change a file and only records the diff. During an upgrade, if there's a "merge conflict", it asks you to reswizzle the component.

I have no idea how heavy the implementation would be on our side, or if there are packages we can use

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

Yes