feat(CategoryTheory/Sums/Basic): functors out of Sum

[robin-carlier]

We prove that the category structure defined in CategoryTheory/Sums/Basic satisfies a universal property that exhibits it as the binary coproduct of categories. In practive, this means that we construct an equivalence of categories (A ⊕ A') ⥤ B ≌ (A ⥤ B) × (A' ⥤ B), which we characterize via its compositions in both directions with Sum.inl_, Sum.inr_, Prod.fst and Prod.snd.

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I was unsure whether or not this should go in a separate new file in the CategoryTheory/Sum directory, as it brings the product as import, but it seems that this file was almost never imported anyways, so that it would be harmless to keep fragmentation to a minimum here.

[github-actions]

PR summary 445a018151

Import changes exceeding 2%

%	File
+2.13%	Mathlib.CategoryTheory.Sums.Basic

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.CategoryTheory.Sums.Basic	282	288	+6 (+2.13%)

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.Sums.Associator Mathlib.CategoryTheory.Sums.Basic	6

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Declarations diff

+ functorEquiv
+ functorEquivFunctorCompFstIso
+ functorEquivFunctorCompSndIso
+ functorEquivInverseCompWhiskeringLeftInLIso
+ functorEquivInverseCompWhiskeringLeftInRIso
+ sum'_app_inl
+ sum'_app_inr
+-+ sum'

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).