[w3c/ServiceWorker] Add new environment settings object field "cross site ancestry" for SameSite cookies work (PR #1775)

To define how cookies work correctly, particularly around the SameSite attribute, we need to know whether or not an environment settings object has any cross-site ancestors. This is making sure the ESO definition in ServiceWorkers also defines this member algorithm. 

We will have to land [the PR that adds this to HTML](https://github.com/whatwg/html/pull/11133) before landing this, but it'd be nice to get this approved before we alter HTML so everything goes smoothly.

See https://github.com/whatwg/html/issues/9000 for even broader context.


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<a href="https://pr-preview.s3.amazonaws.com/bvandersloot-mozilla/ServiceWorker/pull/1775.html" title="Last updated on Jun 3, 2025, 1:30 PM UTC (59ede7d)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/ServiceWorker/1775/ef029c5...bvandersloot-mozilla:59ede7d.html" title="Last updated on Jun 3, 2025, 1:30 PM UTC (59ede7d)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1775

-- Commit Summary --

  * Add new environment settings object field for cookie layering work

-- File Changes --

    M docs/index.bs (2)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1775.patch
https://github.com/w3c/ServiceWorker/pull/1775.diff

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