Add cleanthat to spotless config

[blacelle]

This refers to #1532, considering var usage enforcement.

It does not implement unless the line has a comment part of the spec. I wonder how much this is a strict requirement. (Also, does it include comment before the variable declaration ? What if the variable declaration stands on multiple rows?)

This fails with spotless-gradle 6.15.0 ; the fix is pending for next spotless-gradle release.

[blacelle]

@nedtwigg This has been resynced with main. Ready-for-merging

[nedtwigg]

I am eager to add cleanthat to our code style. But change in our code style will cause merge conflicts, so this PR is also blocked on

• obsolete - Internal-only breaking changes #1096
• obsolete - Add support for linters #1097