chore(SetTheory/Cardinal/Basic): move IsStrongLimit

[vihdzp]

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

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Arguably the Cardinal/Basic file is getting too long, but it seems excessively fragmentary to move IsStrongLimit into its own file. Maybe we can split this file in some other way? e.g. moving all the results on ℵ₀ to an AlephZero file.

[github-actions]

PR summary 6165935b38

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

- IsStrongLimit.ne_zero
- IsStrongLimit.two_power_lt

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.

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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).