Svelte 5: add examples of mixing runes and store to documention

[mheinzerling]

Describe the problem

In most cases, we will not be able to migrate to Svelte 5 and runes in one commit. But even migrating one Writeable to a $state might cause many dependent changes, e.g. in derived. The documentation is missing some hints, how to correctly map/link/convert between old stores and new runes.

I created a small REPL with some mixed use cases. What is missing the get the second button working correctly? (And this the other code 'best practice'?)

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAACpVUS2-jMBD-KyOrByjtpsneSGC1t733sIdlFfGYJFbBRrYJjRD_fcfmkYa2Uvdixp7vMYMHOnbgJWoW_umYSCtkIftZ1-yBmUttN_qMpUHaa9mo3J7sdK54beJEAPCqlspAV6DiZywg1TCGz0YqfIBWcZNmJfZwULKCZNRbaZtO2DYRViaXQhuHRQLvIZp53trfLhD7ySy68fJmOrlCFNNyDxt_dCjRgDapsdp3Nlgou6M3yndj6G1IxRH9D2vFfcVfv1JLcFW59XT8fdoY-dZ2qDWAu1lpYE59TDzdZPsTLwoU18Y-8yDs0uK9zECeXb9Rxt53hp7X-raVT92nkls_EbvVPCS0yeJfsgVq8EjVmxOCRqqugKwxRgpIKcxPqThycXTpoQ9LaKV6oU6UwtyUlx-7VTYoFvxMfpcSo4QVXNdlegmPihdbuzwarOjE4GMuy6YSOoT101P9Ct_nZdivDyphbo6tYAy_p0sF6jXcrezhnO2uV9EvUjagcGxHirzk-UvUeX4UX0lB0McBNeBAA_dWZXPfRqPJNIhLIx3MkOvszaAZ9uxen6ff9-Du6D_Kd_gvVK6j7uYL-rDw5bz34LmxH0esNlwKf5epFQ3ZEk8YgucN_TeqBWF0Gp70o6pkwQ8cCxYa1WD_t_8HCc3hj-MEAAA=

Describe the proposed solution

Add best practice examples for partial migrations.

Importance

would make my life easier

[mheinzerling]

Slightly related to #9287

[dummdidumm]

writable_mixed relies on a store and a state variable, but it will only update when the store value changes, because that's how derived stores work. In general, if you're working with stores and derived stores, don't mix in state variables if you expect them to contribute to the stores updating.

I don't think we need to add this to the documentation, therefore closing this.

[mheinzerling]

@dummdidumm Maybe my sample is kind of artificial. I totally get the point to not to do it in new projects.

Nevertheless, this is of course an actual issue in my migration process. If I migrate a shared writeable to a state rune, I'm forces to update everything that is connected to this writeable in one go. I can't even build the dev server in between to verify the current changes.
Of course anybody could figure out at their own how to address this issue, or decide to reject the migration at all, but a helpful guide in the documentation with some patterns could be useful. If I got Rich right in his last talk, your are interested in moving all project to Svelte 5 and prevent a splitting of the ecosystem.

[dummdidumm]

With "don't need to add this to the documentation" I meant that specific gotcha of using state inside a derived. We'll have documentation about how to migrate, including stores and examples etc.