feat(SetTheory/Ordinal/Basic): recursion on well-orders

[vihdzp]

To define an ordinal-valued function, it suffices to define its values on well-ordered types.

This will be used to redefine the cofinality of an ordinal as the cofinality of a well-order with its order type.

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• depends on: [Merged by Bors] - feat(SetTheory/Ordinal/Basic): notation typeLT α = type (α := α) (· < ·) #18235
• depends on: feat(Order/Basic): PartialOrder.ext_lt #19724

[github-actions]

PR summary 6b388ced5a

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

+ LinearOrder.ext_lt
+ inductionOnWellOrder
+ liftOnWellOrder
+ liftOnWellOrder_type

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).

[mathlib4-dependent-issues-bot]

This PR/issue depends on:

• [Merged by Bors] - feat(SetTheory/Ordinal/Basic): notation typeLT α = type (α := α) (· < ·) #18235
• feat(Order/Basic): PartialOrder.ext_lt #19724
  By Dependent Issues (🤖). Happy coding!