feat(Combinatorics/SimpleGraph/LapMatrix): 0 is always an eigenvalue

[SnirBroshi]

and the determinant is always zero, plus a few other small lemmas.

---

• depends on: [Merged by Bors] - feat(LinearAlgebra/Matrix/Adjugate): M.det = 0 if M *ᵥ v = 0 where v contains a non-zero-divisor #39642

[github-actions]

PR summary 7e35d51b2d

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ IsIsolated.not_isUnit_degMatrix
+ IsRegularOfDegree.degMatrix_eq
+ IsRegularOfDegree.mem_spectrum_adjMatrix
+ degMatrix_bot
+ degMatrix_top
+ lapMatrix_bot
+ lapMatrix_mulVec_apply'
+ lapMatrix_mulVec_one_eq_zero
+ lapMatrix_top
+ not_isUnit_lapMatrix
+ zero_mem_spectrum_lapMatrix
- lapMatrix_mulVec_const_eq_zero

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

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

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

---

No changes to strong technical debt.

No changes to weak technical debt.

[mathlib-dependent-issues]

This PR/issue depends on:

• [Merged by Bors] - feat(LinearAlgebra/Matrix/Adjugate): M.det = 0 if M *ᵥ v = 0 where v contains a non-zero-divisor #39642
  By Dependent Issues (🤖). Happy coding!