Migrate error and warning codes for Svelte 5 support (including in tests)

[Greenheart]

Describe the bug

This is related my comment in #230 about finding out that we likely need to upgrade the compiler + tests to svelte 5. It's good to know that the error + warning codes have changed the separator from - to _.

More details:
https://svelte-5-preview.vercel.app/docs/breaking-changes#other-breaking-changes-error-warning-codes-have-been-renamed

[Greenheart]

The warning missing-declaration seems to have been removed in Svelte 5

For Svelte 5, searching for missing in this page, there is no match: https://github.com/sveltejs/svelte/blob/main/packages/svelte/src/compiler/warnings.js

For Svelte 4, the warning still exists
https://github.com/sveltejs/svelte/blob/d04c146b4649e47d4cb86a93cfa33b910a229591/packages/svelte/src/compiler/compile/compiler_warnings.js#L30

This probably means we need to update the test case checking for this removed compiler warning, and instead trigger another warning. Then we can preserve the intention of the test, but not rely on the changed compiler output in Svelte 5.