Require leading / in paths for modifications

[inexorabletash]

Explainer says that modifications (set, delete) require a leading / in paths:

• https://github.com/WICG/cookie-store/blob/main/explainer.md#paths

Tests validates this:

• https://github.com/web-platform-tests/wpt/tree/master/cookie-store/cookieStore_delete_arguments.tentative.https.any.js
• https://github.com/web-platform-tests/wpt/blob/master/cookie-store/cookieStore_set_arguments.tentative.https.any.js

But the spec seemingly doesn't have anything requiring this:

• https://wicg.github.io/cookie-store/#set-cookie-algorithm

Add it to the spec?

[inexorabletash]

This was fixed.