svelte/internal import guard

[dominikg]

Describe the problem

access to svelte/internal should be limited to blessed code (mostly compiler output).
But for technical reasons it is exported from the svelte package and can be imported by anyone, and a lot of code does: https://github.com/search?q=%22svelte%2Finternal%22+&type=code

Describe the proposed solution

add a guard in resolveId that logs a warning or throws an error upon encountering such an import.

Alternatives considered

maybe eslint-plugin-svelte would also be able to flag these

Importance

nice to have