CSP reportOnly mode

[Rich-Harris]

this is #5078, but without the permission gremlins

Please don't delete this checklist! 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
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

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

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[changeset-bot]

🦋 Changeset detected

Latest commit: 8cc6474

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[Rich-Harris]

i can't technically approve this PR since i'm the 'author', but since @tcc-sejohnson did all the actual work i'm going to abuse my admin privileges and 'approve' it anyway — thanks @tcc-sejohnson!

[Rich-Harris]

oh wait, we just disabled the ability for admins to push directly to master...

[elliott-with-the-longest-name-on-github]

Thanks Rich! I was just in the process of trying to unwind those commits and fix the permissions when I saw this.