EPIC: Block => Fill

[francisrupert]

Summary

Rename --block variants to --fill, e.g. fsa-btn--block => fsa-btn--fill

Tasks

• #547
• #548
• #549
• #546

Background

Many components can be toggled between inline and block/full-width, aka "hug contents" and "fill container" respectively. "Block" traditionally implied "full-width," though is less apt as a variant label now that flex plays a much larger role in things and "block" isn't generic or descriptive enough to explicitly mean "full-width."

As with "variation" changing to "variant" (#530), using "fill" adopts Figma terminology for the same.