feat: definition of a semisimple representation

[stepan2698-cpu]

Defines semisimple monoid representation and registers basic instances. Fixes a universe issue in Maschke.lean.

---

[github-actions]

PR summary 25ea6575e6

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.RepresentationTheory.Semisimple (new file)	1433

---

Declarations diff

+ IsSemisimpleRepresentation
+ instance : IsSemisimpleRepresentation ρ := isSemisimpleRepresentation_of_card_neZero ρ
+ instance [h : IsIrreducible ρ] : IsSemisimpleRepresentation ρ
+ isSemisimpleModule_iff_isSemisimpleRepresentation_ofModule
+ isSemisimpleRepresentation_iff_isSemisimpleModule_asModule
+ isSemisimpleRepresentation_of_card_neZero
+ isSemisimpleRepresentation_of_isIrreducible
+ isSimpleModule_iff_irreducible_ofModule
- is_simple_module_iff_irreducible_ofModule

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).