Add `reassoc` for `LinearMap`

[themathqueen]

Add a reassoc attribute for LinearMap lemmas so that we don't need to reassociate back and forth.
For example, if we have simp lemmas for f ∘ₗ g, then we want it to also apply for f ∘ₗ (g ∘ₗ h).

This was a TODO here:

mathlib4/Mathlib/RingTheory/Coalgebra/Basic.lean

Line 363 in c99f041

-- TODO: Add `reassoc` for `LinearMap`. Then we wouldn't need to reassociate back and forth.

But #34312 golfed it, so that TODO doesn't apply anymore. But I think it's a good thing to do.