v4: Figuring out split dropdown buttons

Fixes #18790 (closed) and changes up #17135 (closed).

This is kinda rough and I'm not super stoked on it, but it's the best idea I've had thus far I think. Here's what's going on (to address the split dropdown button spacing with regards to the caret).

• Added a new functions file to house our strip-unit() function. (In the future this could house a rem-calc function if we wanted one. Note that functions need to be before the variables so that we can use them within the variables file).
• Adds new classes and variables to address the horizontal spacing within split dropdown toggle buttons. This is kinda shit, but it's explicit and is a single class to add. That's better, at least, than using parents and weirdly rules that are specific to markup.
• Update the examples to include these new classes, as well as some sizing examples for the split dropdown buttons since we were lacking those anyway.

Still noodling, and open to more ideas if folks have them.