Rewrite progress component without <progress> element

- <progress> element didn't allow animation, labels overlaid, multiple bars, etc.

- Revamps CSS to use something more similar to v3's implementation

- Ditches variant mixin for `bg-` utils

- Rebuilds docs to match, including adding a new Height section for customizing that.

Only potential remaining todo is adding `.sr-only` instances to within the bar. Unsure if that's necessary.