Refactor: schemas: Separate rule expression type definitions

Rather than one monolithic element-rule definition, create a definition
for each expression type and reference them in the main <choice>. This
will make maintenance easier.

Also cleaned up indentation, since inconsistent indentation often leads
to misreading XML structure.

Ref T196

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>