[rsimp]

This is a really great point. I think something like a `.svelte.js` file extension is warranted here. This would key tooling to when it needs to interpret runes, and makes it clear which files in a codebase require the svelte compiler. These files clearly aren't just js/ts at this point, but I think its fine as long as they're marked as such. Custom react hooks, for instance, aren't usable outside of the runtime but can be transpiled without issues by esbuild/tsc and interpreted correctly by a js/ts language server.

As long as it's marked separately from js/ts I don't think its a huge issue though. Svelte files already have script tags that aren't completely vanilla js/ts.