This will be used to handle stylesheet rendering when we move management
of loading stylesheets to the StyleManager instead of the theme manager.
This sets us up for being able to render specific stylesheets in shadow
roots in addition to just having global stylesheets.
This will take over raw stylesheet management from the theme manager
now that it’s becoming more complex with the need to target specific
host elements. Instead of actually adding nodes to the head of the
document, it will instead simply manage a set of stylesheets we want to
apply and leave actual DOM manipulation to <atom-styles> custom elements
that can render the set of active stylesheets in the appropriate
locations.
