make_block erased the selected objects for every `keep` value except
"retain" and only converted them for "convert", so a misspelt value such
as "retian" removed the objects from the drawing without a reference to
the new block.
`keep` is now parsed into an enum before anything is touched; a value
that is not documented is an error naming the three accepted values. The
documented values are accepted in any case. BLOCK and -BLOCK share the
check.
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
