fix: warn on bidirectional control characters, fix various issues with template expressions

[Ocean-OS]

Closes #6890 by warning if any bidirectional control characters are found in:

• Any part of the source code
• Escaped in a literal or template literal
• A result of a partially evaluated expression

Additionally, I found and fixed a minor bug in the build_template_chunk and process_children functions where certain nullish expressions wouldn't be coallesced to an empty string. I also noticed that the partial evaluation evaluates BigInt calls, which could throw, so I fixed it. Since these are very small changes, I don't think they necessitate a separate PR.

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[changeset-bot]

🦋 Changeset detected

Latest commit: cc7c317

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15893-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15893 

[Rich-Harris]

It's always a good idea to fix separate bugs in separate PRs — in particular, if there are no tests then regressions are very likely

[Ocean-OS]

I see, sorry for the issue. Should I remove the extra code from this PR?

[Rich-Harris]

No need, the merge will take care of it!

[Rich-Harris]

thanks!