Support Svelte 3/4 in playground

[Rich-Harris]

right now we don't respect ?version, we just use latest (it's bundled into the worker). we should fix that, ideally before launch since it will break existing /repl links otherwise

[brunnerh]

@Rich-Harris @dummdidumm This appears to still be broken because the compilation runs in a worker.

There is a function getBtoa which tests for window.btoa and Buffer and finds neither.
globalThis.btoa would be available.

Gets triggered on style source map generation.

Example

<h1>Hello</h1>
<style>h1 { color: red; }</style>

[brunnerh]

Also: The JS output panel appears to still show Svelte 5 output, possibly the other panels as well.

[dummdidumm]

huh, why am I not able to reproduce the btoa issue locally? Very strange.