The title basically says it all. Worth noting that this would initially be beneficial on Paragraph and Heading Blocks as we start adding text-alignment options.

Icon options, depending on if we end up using Dashicons or Material:

• Dashicons: https://developer.wordpress.org/resource/dashicons/#editor-justify
• Material: https://material.io/tools/icons/?search=justify&icon=format_align_justify&style=baseline

Related proposal on Core (not final at the time of this posting): Automattic/wp-calypso#28696