Svelte 5: Warn when whitespace handling would differ from Svelte 4

[Conduitry]

Describe the problem

The changes in whitespace handling between Svelte 4 and Svelte 5 are some of the most insidious breaking changes, in that you may not even notice them until a while after upgrading your app because they may only affect one specific part of it, and possibly in a subtle way.

Describe the proposed solution

It would be nice to (probably just in non-runes mode) have compile-time warnings about components that contain whitespace that is now being removed in Svelte 5 but which was preserved in Svelte 4.

I realize this effectively means reimplementing Svelte 4's insane whitespace rules just for the sake of a warning, but I do think this would help people (myself included) feel more comfortable with migrating.

Importance

nice to have

[gterras]

Could this be directly handled in the migration script as a warning? So that users that don't migrate a Svelte4 codebase don't ever see it?

[Conduitry]

The migration script is intended to help people convert their (still-working) Svelte 4 components into Svelte 5 runes/snippets/etc. syntax.

This change in whitespace handling affects everyone upgrading to Svelte 5, even if their components are still using Svelte 4 syntax, so the compiler seems to me like the right place for this to live, not the migration script.