feat(Combinatorics/SimpleGraph): split Walk.lean into 5 files (#31573)

Split into 5 files in a new `Walks` subfolder:
- `Basic.lean`: basic definitions and theorems about them, without modifying the walks: `Walk`, `Nil`, `length`, `support`, `darts`, `edges`, `edgeSet`
- `Traversal.lean`: access parts of a walk: `getVert`, `snd`, `penultimate`, `firstDart`, `lastDart`
- `Operations.lean`: similar to the existing `WalkDecomp.lean`, this file defines operations on walks: `copy`, `append`, `concat`, `reverse`, `drop`, `take`, `tail`, `dropLast`. The name of the file matches `SimpleGraph/Operations.lean` which defines operations on graphs.
- `Maps.lean`: operations that map the walk to another graph: `map`, `mapLe`, `transfer`, `induce`, `toDeleteEdges`, `toDeleteEdge`. The name of the file matches `SimpleGraph/Maps.lean` which defines maps on graphs.
- `Subwalks.lean`: the definition of `IsSubwalk` and theorems about it

Deprecates the original `Walk.lean` module.

Author: SnirBroshi

Mathlib.lean

@@ -3212,6 +3212,11 @@ public import Mathlib.Combinatorics.SimpleGraph.Turan
 public import Mathlib.Combinatorics.SimpleGraph.Tutte
 public import Mathlib.Combinatorics.SimpleGraph.UniversalVerts
 public import Mathlib.Combinatorics.SimpleGraph.Walk
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Basic
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Maps
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Operations
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Subwalks
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Traversal
 public import Mathlib.Combinatorics.Young.SemistandardTableau
 public import Mathlib.Combinatorics.Young.YoungDiagram
 public import Mathlib.Computability.Ackermann

Mathlib/Combinatorics/SimpleGraph/Connectivity/WalkDecomp.lean

@@ -5,7 +5,7 @@ Authors: Kyle Miller, Pim Otte
 -/
 module
 
-public import Mathlib.Combinatorics.SimpleGraph.Walk
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Operations
 
 /-!
 # Decomposing walks

Mathlib/Combinatorics/SimpleGraph/Paths.lean

@@ -6,7 +6,8 @@ Authors: Kyle Miller
 module
 
 public import Mathlib.Combinatorics.SimpleGraph.Connectivity.WalkDecomp
-public import Mathlib.Combinatorics.SimpleGraph.Walk
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Maps
+public import Mathlib.Combinatorics.SimpleGraph.Walks.Subwalks
 
 /-!