Add an option to set default alignments for image blocks.

Yes, and I think there is a ticket for this (or an issue in Gutenberg repository).
The old editor used to remember what alignment was used last.
To get more attention for this, you can comment on the existing ticket or issue. But you’ll have to find it first… If you don’t find one, write one.
Core: https://core.trac.www.ads-software.com/
Gutenberg: https://github.com/WordPress/gutenberg/issues