feat(SimpleGraph/Connectivity): add vertex reachability and connectivity numbers

[SproutSeeds]

Summary

Adds a first-pass vertex connectivity API for simple graphs.

• define SimpleGraph.isolateVerts
• define SimpleGraph.IsVertexReachable and SimpleGraph.IsVertexConnected
• define SimpleGraph.vertexReachability and SimpleGraph.vertexConnectivity
• add basic lemmas for reflexivity, symmetry, monotonicity, antitonicity, and the k = 0 and k = 1 characterizations

Closes #34960.

Local checks

• ~/.elan/bin/lake build Mathlib/Combinatorics/SimpleGraph/Connectivity/VertexConnectivity.lean
• ~/.elan/bin/lake exe lint-style Mathlib/Combinatorics/SimpleGraph/Connectivity/VertexConnectivity.lean
• bash scripts/mathlib-issue-local-gate.sh --repo /Users/codymitchell/Documents/code/mathlib4 --issue 34960 --build-target Mathlib/Combinatorics/SimpleGraph/Connectivity/VertexConnectivity.lean --lint-module Mathlib.Combinatorics.SimpleGraph.Connectivity.VertexConnectivity --skip-mk-all

[github-actions]

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[github-actions]

PR summary 3426e8e0f1

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Combinatorics.SimpleGraph.Connectivity.VertexConnectivity (new file)	778

---

Declarations diff

+ IsVertexConnected
+ IsVertexConnected.connected
+ IsVertexConnected.preconnected
+ IsVertexConnected.zero
+ IsVertexReachable
+ IsVertexReachable.anti
+ IsVertexReachable.mono
+ IsVertexReachable.reachable
+ IsVertexReachable.refl
+ IsVertexReachable.symm
+ IsVertexReachable.zero
+ isVertexConnected_one
+ isVertexReachable_comm
+ isVertexReachable_one
+ isolateVerts
+ isolateVerts_empty
+ isolateVerts_le
+ isolateVerts_mono
+ le_vertexConnectivity
+ le_vertexReachability
+ vertexConnectivity
+ vertexConnectivity_le_vertexReachability
+ vertexReachability

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/reporting/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).

[SproutSeeds]

Thanks for flagging this. You are right that this overlaps with #33355, and I should have coordinated first. I am closing this PR to avoid duplicate review load. Sorry for the noise here. I will help on #33355 instead and coordinate there.