feat(Nat/Digits/Lemma): add a lemma to unfold Nat.digitsAppend

[xroblot]

As suggested on Zulip

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[github-actions]

PR summary 39046b0451

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

+ digitsAppend_def

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

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

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.

[github-actions]

✅ PR Title Formatted Correctly

The title of this PR has been updated to match our commit style conventions.
Thank you!

[ocfnash]

Although I agree with Riccardo's [remark on Zulip](#mathlib4 > expose missing in `Mathlib.Data.Nat.Digits.Lemmas`? @ 💬), I'm not yet convinced that this is the right lemma to add here. I would regard the RHS of this lemmas as an implementation detail and I think we might do better just to have API to characterise this function.

Can you give an example of why you want this?

[xroblot]

Although I agree with Riccardo's [remark on Zulip](#mathlib4 > expose missing in Mathlib.Data.Nat.Digits.Lemmas? @ 💬), I'm not yet convinced that this is the right lemma to add here. I would regard the RHS of this lemmas as an implementation detail and I think we might do better just to have API to characterise this function.

Can you give an example of why you want this?

Yeah, I agree that unfolding this definition for some use outside of this file does not make much sense. A better solution is probably to eventually add the results and/or API that I need directly to this file. I'll close this PR.