This patch:
* adds some simple logic to settings search that should prevent us from having to manually allow searching in every moz- prefixed custom element
* cleans up the existing settings search logic slightly for improved readability
* expands the tests to check searching for text in the shadow DOM of additional moz- prefixed custom elements
Differential Revision: https://phabricator.services.mozilla.com/D214141
f26d450f6e
