feat: Improve errors from dev-time illegal module imports

[elliott-with-the-longest-name-on-github]

There are some (rightful) complaints that the errors we provide are super unhelpful:

• Closes "Cannot import into client-side code" errors are incredibly difficult to debug #13261
• Closes "Cannot import $env/static/private.js into client-side code" when importing from a common functions file #5937

Neither of these describes an actual bug, but they do show some frustration. Because we don't have knowledge of the full module graph during devtime, the error messages are kinda nasty and unhelpful, especially when we can't even provide information about the importer. This adds some information to the error to make it easier to understand. New errors will look like this:

Cannot import ${illegal_module} into client-side code. This could leak sensitive information.`
{if importer is available: Imported by ${importer}.}
Tips:
 - To resolve this error, ensure that no exports from ${illegal_module} is not used, even transitively, in client-side code.
 - If you're only using the import as a type, change it to \`import type\`.
 - If you're not sure which module is causing this, try building your app -- it will create a more helpful error.

---

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13329-svelte.vercel.app/

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adjusted

[dummdidumm]

Needs to adjust the test output, then this is good to go