feat(GroupTheory/GroupAction): instance MulAction (β ⧸ H) α

[shuxuezhuyi]

We instance this naturally from MulAction β α when H is a normal subgroup and acts trivially on α.
We also instance IsometricSMul (M ⧸ N) X in this way.

---

[YaelDillies]

I think @eric-wieser already did something similar.

[urkud]

Does it create diamonds for the action of G ⧸ H on itself? On the one hand, we have the regular action. On the other hand, we can use your instance to go from the action of G. While these two actions seem to be equal (and probably defeq), I'm not sure that they are reducibly defeq. @eric-wieser may have more insight about this.

[urkud]

@eric-wieser Could you please have a look?

[grunweg]

@shuxuezhuyi Coming here for PR triage: thanks for your PR. It currently has a merge conflict, which means it won't show up in the review queue (and is less likely to be noticed). Can you merge the master branch, please? If you would like help with this, feel free to ask on zulip.

[github-actions]

PR summary 8eb42b45f9

Import changes exceeding 2%

%	File
+2.23%	Mathlib.Topology.MetricSpace.IsometricSMul

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Topology.MetricSpace.IsometricSMul	1075	1099	+24 (+2.23%)
Mathlib.GroupTheory.GroupAction.Quotient	730	733	+3 (+0.41%)

Import changes for all files

Files	Import difference
Mathlib.LinearAlgebra.Alternating.DomCoprod Mathlib.Algebra.Polynomial.GroupRingAction	1
4 files Mathlib.GroupTheory.FreeGroup.NielsenSchreier Mathlib.CategoryTheory.Action Mathlib.GroupTheory.GroupAction.CardCommute Mathlib.GroupTheory.GroupAction.Quotient	3
Mathlib.Analysis.Normed.Group.AddTorsor	19
Mathlib.Topology.MetricSpace.IsometricSMul	24

---

Declarations diff

+ QuotientGroupAction
+ QuotientGroupAction.isometricSMul
+ _root_.AddAction.QuotientAddGroupAction
+ mulAction
+ smul

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).

[YaelDillies]

Can you tell us your use case?