feat: add support and docs for kirbyup serve

[jonaskuske]

⚠️ depends on johannschopplich/kirbyup#18 and getkirby/kirby#4541

[lukasbestle]

Thanks for the PR! Sorry, I missed it and updated the package.json already. But we can still update it again.

I added a new command serve instead of changing the dev command. My thought was that this allows a migration path for devs who develop plugins that need to support older Kirby versions and also for devs who don't want to live on the bleeding beta edge.

My suggestion for the docs (new paragraph in the "Panel reloading" section) is this:

<since v="3.7.4">
kirbyup 2.0.0 (currently in beta) introduces a new `serve` command that allows hot module reloading. This feature will automatically reload the Panel if you make changes to your plugin. The new `serve` command requires Kirby 3.7.4+.

You can use the `serve` command instead of `npm run dev`:

``
npm run serve
``

You can pass [additional options](https://github.com/johannschopplich/kirbyup/tree/main#kirbyup-serve-input) if needed by updating the command in `package.json` or you can pass them manually like this:

``
npm run serve -- --port 1234
``
</since>

[johannschopplich]

It will smoothen the transition for current plugin devs. A nice addition in my opinion. I have changed the default dev command to stay as in kirbyup v1 and added the serve command for the new feature.