fix(material/core): M3 themes not inserting loaded marker

[crisbeto]

Fixes that all M3 themes were causing a "no theme has been loaded" warning to be logged, because they weren't inserting the loaded marker.

Note: it's tempting to create the marker as a token, but we can't do it because tokens are output under a selector, whereas we want the marker to always be at the top level since we detect it by creating an element and inserting it into the body.

Fixes #29115.