feat: improve playground ux + migrate to svelte 5

[ottomated]

This migrates components from the svelte 5 playground to use svelte 5 and makes a few visual / functional tweaks. For example (left/right = before/after):

Svelte 5 rewrite

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

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• Run the tests with pnpm test and lint the project with pnpm lint

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[ottomated]

@Rich-Harris Had to patch @rich-harris/svelte-split-pane to allow it to return it's current position (the repl was trying to bind to it when it wasn't bindable, which caused a bunch of jank).

[Rich-Harris]

Thank you, but I wish you had mentioned this somewhere before taking this on — the current playground is more or less on ice as we're working on an entirely new svelte.dev site. It's not open source yet, but it already has a bunch of Svelte 5 playground updates, and we're trying to minimise how much of a clusterfuck it'll be to port over changes from the preview site (otherwise there's a whole bunch of changes I'd have made to the preview playground already)

[ottomated]

I'll hold off on the more complicated sourcemap-related stuff then and stick to low-hanging fruit 👍

[Rich-Harris]

What I meant was we're not going to merge this PR - we can leave it open for reference when working on svelte.dev but beyond that it would be too disruptive

[ottomated]

What I meant was we're not going to merge this PR - we can leave it open for reference when working on svelte.dev but beyond that it would be too disruptive

Sounds good, I'd be interested in contributing to the new site once it is open.