feat: demos on Glitch

[hirasso]

Closes #183
Also discussed here: https://github.com/orgs/swup/discussions/967

Description

• Migrate demos from Replit to Glitch
• Update embed code accordingly
• Intentionally keeping the implementation as similar as possible to the previous replit embeds (code fences)

Direkt Link to Deploy Preview

TODO

• Port all demos

Drive-By

• added SwupPreloadPlugin to a few of the demos (especially the ones for parallel plugin) to make them more snappy

Checks

• The PR is submitted to the master branch
• The code was linted before pushing (npm run lint)

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[daun]

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[daun]

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