feat(Order/PartialSups): add exists_partialSups_eq

[lua-vr]

In a linear order, there exists an index j ≤ i for which partialSups f i = f j.

---

Adapted from a proof generated by Aristotle.

[github-actions]

PR summary 721b21cf2b

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Order.PartialSups	511	512	+1 (+0.20%)

Import changes for all files

Files	Import difference
Mathlib.Algebra.Order.Group.PartialSups Mathlib.Order.PartialSups	1

---

Declarations diff

+ exists_partialSups_eq

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.

---

No changes to technical debt.

You can run this locally as

./scripts/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).