feat(RingTheory/MvPolynomial): MvPolynomial is isomorphic to the direct sum of its homogeneous submodules

[mpenciak]

This PR closes a TODO in Mathlib/RingTheory/MvPolynomial/Homogeneous.lean.

It defines the equivalence MvPolynomial σ R ≃ₐ[R] ⨁ i, homogeneousSubmodule σ R i, and the more general statement for weighted homogeneous submodules.

Note: I'm not sure this is actually worth merging, as it's basically already contained in the weightedGradedAlgebra typeclass... But having this exact statement could be helpful so it could go either way?

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[github-actions]

PR summary 90f65323f6

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

+ algEquivDirectSumHomogeneousSubmodule
+ algEquivDirectSumWeightedHomogeneousSubmodule

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 scripts/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).