First pass at adding box-shadow utilities with .shadow-* classes. Right now these are pulled from other components and modified slightly. They're also conditionally included based on the $enable-shadows boolean variable. Still need to finish docs and update existing shadow values using these new variables.
Update: I've removed the $enable-shadows conditional, meaning these variables are always available now. I've also added a .shadow-none option.
Fixes #23437 (closed).