feat: add List.Pairwise.orderedInsert', List.sortedLE_orderedInsert_LT, and List.sortedGE_orderedInsert_GT

[jacob-greenfield]

Introduce a generalized Pairwise.orderedInsert' lemma to prove that Pairwise r l is maintained after performing an orderedInsert using a relation stronger than r under certain conditions. Prove sortedLE_orderedInsert_LT, showing that l.orderedInsert (· < ·) a preserves SortedLE for decidable total preorders (analogously for sortedGE_orderedInsert_GT). This is useful for inserting an element into a sorted list behind any other elements which compare equal.

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I was hoping to avoid omit, but that would require moving all of Pairwise.orderedInsert', Pairwise.orderedInsert, pairwise_insertionSort, sublist_insertionSort', pair_sublist_insertionSort', and mergeSort_eq_insertionSort after end sort on line 365 and reintroducing all the section variables. I also considered moving Pairwise.orderedInsert' somewhere before variable {α β : Type*} (r : α → α → Prop) (s : β → β → Prop) on line 30, but that would place it before orderedInsert is defined. "Omit" seemed like the least invasive approach.

[github-actions]

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[github-actions]

PR summary 33867085fb

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff (regex)

+ Pairwise.orderedInsert'
+ sortedGE_orderedInsert_GT
+ sortedLE_orderedInsert_LT

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.

Declarations diff (Lean -- unavailable)

⚠️ No declarations diff yet: there is no built master snapshot at this PR's merge-base (typically a bors-batch intermediate that CI never built). Merge master into this PR and push to refresh.

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit 33867085fb
Reference commit 1f29011071

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/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).