feat(category_theory/enriched): abstract enriched categories (#7175)
# Enriched categories
We set up the basic theory of `V`-enriched categories,
for `V` an arbitrary monoidal category.
We do not assume here that `V` is a concrete category,
so there does not need to be a "honest" underlying category!
Use `X ⟶[V] Y` to obtain the `V` object of morphisms from `X` to `Y`.
This file contains the definitions of `V`-enriched categories and
`V`-functors.
We don't yet define the `V`-object of natural transformations
between a pair of `V`-functors (this requires limits in `V`),
but we do provide a presheaf isomorphic to the Yoneda embedding of this object.
We verify that when `V = Type v`, all these notion reduce to the usual ones.
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>