fix(repl): update broken embed repl to work like regular one

[zusorio]

I noticed that the /repl/embed route on svelte.dev is currently broken.
This PR fixes it and also moves it to be more in line with how the normal REPL works.
I've moved the route from /repl/embed?gist={id} to /repl/[id]/embed and set up a redirect.

Because of this, ReplWidget was only being used for the examples on the landing page, so I replaced it and removed ReplWidget.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

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

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Latest commit: c41fff0

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[PuruVJ]

This looks amazing, thanks a lot!! Im travelling atm, so will test it out tomorrow and merge

[zusorio]

Hey, I just wanted to ask if this PR will get merged before next Tuesday? I'm planning on using the embedded REPL in a presentation and wanted to know if I should set up my own deployment of the svelte REPL or if it'll be ready by then. If not that's totally fine, I'd just like to know so I can plan.

[PuruVJ]

Sure, will merge by sunday latest!