chore(ring_theory/derivation): split file #19138

Stream: PR reviews

[Scott Morrison]

Is a straightforward split not that far ahead of the tide, hopefully someone can have a look soon.

It is not particularly important for the port itself; rather I was showing a non-Lean mathematician (gasp) how the port is going and they correctly complained about the Stone-Weierstrass theorem "depending on" Lie algebras.