[RFC] Svelte 5 (@next) support

[MentalGear]

How do you envision this feature/change to look/work like?

Svelte 5 (svelte@next) brings major productivity and code unification improvements, among other things allowing to use reactivity in any js/ts file.

I tried using svelte@next with plasmo, but plasmo errors out saying that the frontend is not supported.

What is the purpose of this change/feature? Why?

Add svelte@next to list of allowed frontends to be used with plasmo.

(OPTIONAL) Example implementations

No response

(OPTIONAL) Contribution

• [X] I would like to contribute to this RFC via a PR

Verify canary release

• [X] I verified that the issue exists in plasmo canary release

Code of Conduct

• [X] I agree to follow this project's Code of Conduct
• [X] I checked the current issues for duplicate problems.

[braden-w]

Same here. Tried to upgrade my extension to Svelte 5 and was getting a ton of issues. Eventually, I realized that even a skeleton project that is Svelte 5 rather than Svelte 4 would give the following error:

🔴 ERROR | No supported UI library found. You can file an RFC for a new UI Library here: https://github.com/PlasmoHQ/plasmo/issues