Overlay form controls, selects, and files with icons or spinners

This is a draft of a new set of options for our .form-control, .form-select, and .form-file to add icons and spinners. I've included support for sizing, too. Had to rearrange some things though, including how we're calculating the input height variables (they weren't right, now they are), but otherwise this was relatively straightforward.

Questions, ideas, and todos:

• Do we need to update this to support icons on the left and right? If so, do we use start and end over right and left in the names? This is mentioned here: https://github.com/twbs/bootstrap/pull/29107#issuecomment-575000676.
• Do we split the Sass here, perhaps with a mixin, across each file?
• Or maybe we put this as a separate page altogether?
• Any other variations we need to support? Perhaps something on focus to change the icon color?
• Do we implement this in v4 as well? Would be relatively straightforward I think, but would be a minor bump.
• Anything else?

Replaces #29107. Fixed #28600.