chore(topology/category/Top/limits): split file (#18871)
Per
https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/!4.233487.20last.20minute.20split.3F
This file is already being ported at https://github.com/leanprover-community/mathlib4/pull/3487, but:
* it's not going so well right now
* it is going well up to the point of the proposed new `limits/basic.lean`
* that is sufficient to port the files needed for Copenhagen
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
