feat(AlgebraicTopology/SimplexCategory/GeneratorsRelations/NormalForms): admissible lists and simplicial insertion

[robin-carlier]

We introduce the notion of an m-admissible list of natural number: a list [i₀,…,iₙ] is said to be m-admissible if iₖ ≤ m + k for all valid index k. These lists are intended to uniquely represents either P_σ or P_δ morphisms in SimplexCategoryGenRel.
We prove basic stability of such lists under some lists operation such as the tail.
We also introduce the notion of simplicial insertion, which is intended to be the algorithm on such normal forms representing composition on the right (for P_σ morphisms) or on the left (for P_δ morphisms) by a single morphism, and we prove that this preserves admissibility.

Part of a series of PR formalising that SimplexCategoryGenRel is equivalent to SimplexCategory.

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• depends on: [Merged by Bors] - feat(AlgebraicTopology/SimplexCategory/GeneratorsRelations/EpiMono): epi-mono factorisation in SimplexCategoryGenRel #21743

[github-actions]

PR summary 44a40ae83f

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.AlgebraicTopology.SimplexCategory.GeneratorsRelations.NormalForms (new file)	615

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Declarations diff

+ IsAdmissible
+ cons
+ ext
+ getElemAsFin
+ head
+ head_lt
+ le
+ nil
+ simplicialInsert
+ simplicialInsert_isAdmissible
+ simplicialInsert_length
+ sorted
+ tail

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.

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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).

[mathlib4-dependent-issues-bot]

This PR/issue depends on:

• [Merged by Bors] - feat(AlgebraicTopology/SimplexCategory/GeneratorsRelations/EpiMono): epi-mono factorisation in SimplexCategoryGenRel #21743
  By Dependent Issues (🤖). Happy coding!