feat(PowerSeries): rename a power series as multivariate power series

[WenrongZou]

---

In this PR, I define the PowerSeries.toMvPowerSeries which use Polynomial.toMvPolynomial as reference. Also there is some missing APIs in MvPowerSeries/Equiv.lean with respect to MvPolynomial/Equiv.lean. I will try to add them in next several PRs.

• depends on: feat(MvPowerSeries): connection between rename and subst #37576

[github-actions]

PR summary 389a421660

Import changes exceeding 2%

%	File
+10.93%	Mathlib.RingTheory.MvPowerSeries.Equiv
+27.19%	Mathlib.RingTheory.MvPowerSeries.Rename

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.RingTheory.MvPowerSeries.Rename	1129	1436	+307 (+27.19%)
Mathlib.RingTheory.MvPowerSeries.Equiv	1583	1756	+173 (+10.93%)

Import changes for all files

Files	Import difference
Mathlib.RingTheory.MvPowerSeries.Equiv	173
Mathlib.RingTheory.MvPowerSeries.Rename	307

---

Declarations diff

+ HasSubst.X_comp
+ HasSubst.toMvPowerSeries
+ MvPowerSeries.rename_comp_toMvPowerSeries
+ MvPowerSeries.rename_toMvPowerSeries
+ _root_.MvPowerSeries.monomial_mapDomain_eq_prod
+ rename_eq_subst
+ toMvPowerSeries
+ toMvPowerSeries_C
+ toMvPowerSeries_X
+ toMvPowerSeries_apply
+ toMvPowerSeries_eq_subst
+ toMvPowerSeries_injective
+ toMvPowerSeries_val

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/reporting/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).

[mathlib-dependent-issues]

This PR/issue depends on:

• feat(MvPowerSeries): connection between rename and subst #37576
  By Dependent Issues (🤖). Happy coding!

[mathlib-merge-conflicts]

This pull request has conflicts, please merge master and resolve them.