feat(Algebra): add missing substructure lemmas

[artie2000]

Add missing boilerplate lemmas about the maps .toNonUnitalSubsemiring and .subsemiringClosure

This PR is split off from #16094

---

[github-actions]

PR summary 8193df3116

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ Submonoid.subsemiringClosure_toNonUnitalSubsemiring
+ Subsemiring.coe_toNonUnitalSubsemiring
+ Subsemiring.mem_toNonUnitalSubsemiring
+ Subsemiring.toNonUnitalSubsemiring_eq
+ Subsemiring.toNonUnitalSubsemiring_injective
+ subsemiringClosure_mem

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]

Thanks!

maintainer delegate

[YaelDillies]

Suggested change

	theorem Subsemiring.toNonUnitalSubsemiring_eq {S₁ S₂ : Subsemiring R} :
	theorem Subsemiring.toNonUnitalSubsemiring_inj {S₁ S₂ : Subsemiring R} :

[artie2000]

named by analogy to e.g. Subgroup.toSubmonoid_eq

[artie2000]

ah, many precedents exist for both conventions
is there consensus on which is preferred?

[github-actions]

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