feat(Positivity): add support for positivity [h₁, h₂] syntax

[HugLycan]

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This PR adds support for positivity [h₁, h₂] syntax.
related issue #2427
@fpvandoorn

[github-actions]

PR summary 06bd309675

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

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.

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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).

[grunweg]

Thank you for working on this; I have wanted this feature for a while!

It would be nice to document this a bit better, so users can learn what this does by just reading the documentation. (I'm also very curious to see how much of mathlib could be simplified with this, but that should probably happen in a separate pull request.)

[grunweg]

Looks good to me, with one tiny tweak. @fpvandoorn Would you like to take a final look?

[HugLycan]

-awaiting-author

[thorimur]

If you would, please add a simple test for positivity []; see other comment. :)