refactor: switch from .ts to .js with jsdoc

[dominikg]

This PR is part 1 of a 2 part process.

• Change all types to use jsdoc in vite-plugin-svelte, but keep .ts extension.
• Cleanups and changes to tooling as needed

Part 2 in a separate PR:

• rename .ts to .js
• update package.json to only include src, point to src/index.js / d.ts
• change build script to be typecheck only (we can remove it later but it is called from ecosystem-ci etc)
• add changeset