feat(CategoryTheory): Guitart exact squares and Kan extensions

[joelriou]

Given a Guitart exact square w : T ⋙ R ⟶ L ⋙ B, we show that pointwise left Kan extensions along R give, by composition, pointwise left Kan extensions along L.

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[github-actions]

PR summary cc0d5cbbe4

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.GuitartExact.KanExtension (new file)	638

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Declarations diff

+ IsPointwiseLeftKanExtension.compTwoSquare
+ compTwoSquare
+ hasColimit_comp_iff
+ hasLeftKanExtension
+ hasLimit_comp_iff
+ hasPointwiseLeftKanExtension
+ hasPointwiseLeftKanExtensionAt_iff
+ hasPointwiseLeftKanExtension_iff
+ instance (Y : D) : Subsingleton (E.IsPointwiseLeftKanExtensionAt Y)
+ instance (Y : D) : Subsingleton (E.IsPointwiseRightKanExtensionAt Y)
+ isPointwiseLeftKanExtensionAtCompTwoSquareEquiv
+ isPointwiseLeftKanExtensionEquivOfGuitartExact
+ isPointwiseLeftKanExtensionOfCompTwoSquare
+ nonempty_isPointwiseLeftKanExtensionAt_compTwoSquare_iff

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).