feat(Topology/Separation/Hausdorff): add Filter.limUnder_congr

[emlis42]

This PR adds Filter.limUnder_congr and some lemmas about limUnder.

Filter.limUnder_congr allows rewriting limUnder expressions using equivalences of convergence behavior, without proving the filters actually converge, allowing convergence proofs to be carried out on more convenient expressions latter.

[github-actions]

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[github-actions]

PR summary 0f4efd907f

Import changes exceeding 2%

%	File
+13.16%	Mathlib.Order.Filter.AtTopBot.Archimedean

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Order.Filter.AtTopBot.Archimedean	798	903	+105 (+13.16%)

Import changes for all files

Files	Import difference
Mathlib.Order.Filter.AtTopBot.Interval	94
Mathlib.Order.Filter.AtTopBot.Archimedean	105

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Declarations diff

+ Filter.limUnder_congr
+ PNat.limUnder_eq
+ PNat.limUnder_eq_limUnder_succ
+ limUnder_add_eq_limUnder_nat

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.