Fix TypeScript errors in view-transitions.mdx code snippets

[ArmandPhilippot]

Description (required)

Follow-up of #14089. TL;DR: Improve UX by preventing the appearance of red squiggles (TS) and visual changes (formatting with Astro extension) when the user copies and pastes the code snippet.

Fixes code snippets in guides/view-transitions.mdx:

• add missing Astro.props
• querySelector can return null
• event.target.value is not available without narrowing the type
• redirect can be null when using params.get
• unclosed <script /> tag
• consistency: other snippets are wrapped in script tags
• setupStuff() comes from nowhere, a fake import can be helpful

I haven't updated a few code snippets in view-transitions.mdx as I'm not really familiar with the API but:

• https://docs.astro.build/en/guides/view-transitions/#fallback-control a transition on <title> seems weird to me
• https://docs.astro.build/en/guides/view-transitions/#astrobefore-swap the second code snippet uses diff() in an is:inline script... not sure this is clear to everyone where diff() comes from.

Related issues & labels (optional)

• Suggested label: code snippet update

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en/guides/view-transitions.mdx	Source changed, localizations will be marked as outdated.

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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