warning: mathlib reorganization

Stream: Infinity-Cosmos

[Nick Ward]

Small PSA: mathlib4#19463 (merged) and mathlib4#19472 (merged) do some minor reorganizing of quasicategory-related files.

In particular, mathlib4#19463 adds a directory AlgebraicTopology/Quasicategory/ and moves AlgebraicTopology/SimplicialSet/Quasicategory.lean to AlgebraicTopology/Quasicategory/Basic.lean.

mathlib4#19472 breaks out the quasicategory-dependent material from AlgebraicTopology/SimplicialSet/StrictSegal.lean into two new files, AlgebraicTopology/Quasicategory/StrictSegal.lean and AlgebraicTopology/Quasicategory/Nerve.lean.

I apologize in advance for the merge conflicts, and I'm happy to help resolve them.

[Emily Riehl]

@Nick Ward will you let me know when #19472 is merged?

[Johan Commelin]

[Nick Ward]

@Emily Riehl merged!

[Emily Riehl]

And now we've reorganized ForMathlib within InfinityCosmos similarly. This will likely break things in any forks of our repository.