feat(CategoryTheory): Define subobject classifier

[CharredLee]

This creates the folder CategoryTheory/Topos and the file CategoryTheory/Topos/Classifier.lean. The proposed code includes the definition of what it means to be a subobject classifier in a category C with a terminal object, and relevant basic results about such a category. This is done in the "internal" sense, for the purpose of eventually defining what it means for a category to be a topos utilizing this definition; see also #21269.

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[github-actions]

PR summary fc487e3a9f

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.Topos.Classifier (new file)	602

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Declarations diff

+ Classifier
+ HasClassifier
+ comm
+ isPullback_χ
+ isRegularMonoCategory
+ reflectsIsomorphisms
+ reflectsIsomorphismsOp
+ truth
+ truthIsRegularMono
+ unique
+ Ω
+ χ

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).

[CharredLee]

Note that the reason for not putting this in the folder CategoryTheory/Subobject is precisely because it uses the "internal" definition. A future extension of this content will be to define CategoryTheory/Subobject/Classifier.lean and show that the "external" definition of a subobject classifier (that the Sub functor is representable) is equivalent.

[b-mehta]

I'm happy with the maths here, just style and naming questions now. We briefly discussed how the limit assumptions should be set up, I think we agreed that the one here is sensible for now?