[Merged by Bors] - feat(Algebra/Star/NonUnitalSubsemiring): Add NonUnitalStarSubsemiring

[mans0954]

Definitions and basic properties of non-unital star subsemirings

---

This file heavily copies Mathlib.Algebra.Star.NonUnitalSubalgebra, so I'm happy to assign the copyright to @j-loreaux if that's the appropriate thing to do.

So far I've mostly only developed what's required for #6595 - I thought I'd check that this was the right approach before going further.

[j-loreaux]

I think we don't want StarSubset, unless you have an explicit application in mind. Also, it would be nice if the unital versions of all these were placed in a separate file.

[j-loreaux]

no need to include me in the copyright.

[github-actions]

Import summary

No significant changes to the import graph

[github-actions]

PR summary f3295abb61

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Algebra.Star.NonUnitalSubsemiring	572
Mathlib.Algebra.Star.Subsemiring	584

---

Declarations diff

+ NonUnitalStarSubsemiring
+ StarSubsemiring
+ SubStarSemigroup
+ coe_mk
+ coe_toSubsemiring
+ ext
+ instNonUnitalSubsemiringClass
+ instSetLike
+ instStarMemClass
+ mem_toSubsemiring
+ semiring
+ setLike
+ starMemClass
+ starRing
+ subsemiringClass
+ toSubsemiring_inj
+ toSubsemiring_injective
+ toSubsemiring_le_iff
++ coe_copy
++ copy
++ copy_eq
++ mem_carrier
+++ center

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>

[j-loreaux]

bors d+

[mathlib-bors]

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[mans0954]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

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