The intent of this patch is to add support for feature callout anchors situated in the shadowDOM. For instance, for the Review Checker migration project, we want to add callout messages on the feature's sidebar icon. The actual parsing of the selector itself should be credited to @aminomancer; I'm authoring this patch to make sure we can get the change landed and tested.
I also verified the change with a few selectors via about:asrouter. For instance, if you wish to test with the actual Review Checker sidebar icon, you should have the prefs `sidebar.revamp` and `browser.shopping.experience2023.integratedSidebar` set to true. After enabling RC in the sidebar settings, add the following anchor to any callout: `"#sidebar-main > sidebar-main::%shadow% .tools-and-extensions::%shadow% moz-button[view='viewReviewCheckerSidebar']"`.
Lastly, I added a test in `browser/components/asrouter/tests/browser/browser_feature_callout_panel.js` to ensure `%shadow%` works as intended.
Differential Revision: https://phabricator.services.mozilla.com/D231337
c8197f60d5