Prove that regular monomorphisms are stable under pullbacks

This is [Category.Limit/regularMono_pullback](https://github.com/JetBrains/arend-lib/blob/4ea97da483c5bb2e6a22c8d09c03becf17756c7e/src/Category/Limit.ard#L591). This should be straightforward.

Open src/Category/Limit.ard at the Category.Limit/regularMono_pullback entry around line 591 and read the surrounding definitions and existing limit proofs. Establish the regular-monomorphism pullback result there, then verify that the Arend library checks successfully and the theorem is accepted.

Written by the indexing model from the issue text.