[Merged by Bors] - chore(SetTheory/Cardinal/Basic): move IsStrongLimit

[vihdzp]

This definition doesn't rely on cofinality in any way.

---

• depends on: [Merged by Bors] - chore(SetTheory/Cardinal/Cofinality): make IsStrongLimit into a structure #21971

Arguably the Cardinal/Basic file is getting too long, but it seems excessively fragmentary to move IsStrongLimit into its own file.

[github-actions]

PR summary d9296768ec

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

No declarations were harmed in the making of this PR! 🐙

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).

[mathlib4-dependent-issues-bot]

This PR/issue depends on:

• [Merged by Bors] - chore(SetTheory/Cardinal/Cofinality): make IsStrongLimit into a structure #21971
  By Dependent Issues (🤖). Happy coding!

[YaelDillies]

maintainer merge

Are you eventually planning to split Cardinal.Basic?

[github-actions]

🚀 Pull request has been placed on the maintainer queue by YaelDillies.

[vihdzp]

Yeah, it's one of the many things I plan to eventually get to.

[kim-em]

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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