feat: Sym2-as-Finset theory

[kmill]

---

[github-actions]

PR summary 8c08565c6e

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ card
+ card_toFinset_mk_of_ne
+ card_toFinset_of_isDiag
+ card_toFinset_of_not_isDiag
+ coe_toFinset
+ edge_toFinset_card
+ isDiag_iff_exists
+ mem_toFinset
+ setOf_mem_eq
+ toFinset

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.

[FordUniver]

I just noticed that there is both the file Mathlib.Data.Sym.Sym2.lean and the folder Mathlib.Data.Sym.Sym2. Is this intended or should the contents of Sym2.lean eventualy migrate into the folder?