Support latest Svelte features without modern AST

[Andreas02-dev]

Describe the problem

At StrykerJS, we want to keep support for Svelte 4 while also introducing support for Svelte 5. Since we don't bundle the Svelte compiler (bring-your-own-compiler) and still want to support Svelte 4, we cannot make use of the modern AST just yet.

Describe the proposed solution

We would like to know the following:

• Are any new features introduced in Svelte 5 incompatible when making use of the legacy AST?
• Will the legacy AST be supported and actively maintained until Svelte 7 is released (so throughout the lifetimes of Svelte 5 and 6)?

Importance

nice to have

[dummdidumm]

Yes, the legacy AST gets all the new features (you can see for yourself that e.g. svelte:boundary has an AST node). We're not sure yet at which point the old AST is removed, but it will not be during Svelte 5.