[CI] Sign PowerAccent.Common.dll (new managed library from #47211) (#48058)

## Summary

Fixes the **stable** signed release pipeline failure (Dart build
[`147621162`](https://microsoft.visualstudio.com/Dart/_build/results?buildId=147621162))
at the *"Verify all binaries are signed and versioned"* task:

```
Not Signed: + …\extractedMachineMsi\File\BaseApplicationsFiles_File_PowerAccent.Common.dll
```

## Root cause

PR #47211 (*"[Quick Accent] Move language data to PowerAccent.Common
library, refactor"*) introduced the new managed library:

- `src/modules/poweraccent/PowerAccent.Common/PowerAccent.Common.csproj`

`PowerAccent.Common.dll` is referenced by both `PowerAccent.Core` (ships
in the installer root) and `PowerToys.Settings` (ships in WinUI3Apps;
deduplicated against root by `generateAllFileComponents.ps1`). The DLL
is harvested into the MSI's `BaseApplicationsFiles` component group, but
the PR did not update `.pipelines/ESRPSigning_core.json`, so ESRP never
signs it and `versionAndSignCheck.ps1` correctly fails the build.

This is the same kind of omission that PR #48050 fixed for
`YamlDotNet.dll` introduced by #40834 — a new managed library shipping
in the MSI without a matching signing config entry.

## Fix

One additive line in `.pipelines/ESRPSigning_core.json`, placing
`"PowerAccent.Common.dll"` inside the existing alphabetized PowerAccent
block (next to `PowerAccent.Core.dll`).

```diff
             "PowerAccent.Core.dll",
+            "PowerAccent.Common.dll",
             "PowerToys.PowerAccent.dll",
```

## Validation

- `git diff` shows exactly one additive line.
- File still parses as valid JSON (`ConvertFrom-Json` round-trip OK).
- Audited the full file list of PR #47211: `PowerAccent.Common.dll` is
the only new shippable assembly it added (the other new project,
`PowerAccent.Common.UnitTests`, is a test project and is not included in
the installer).
- Dedup logic in
`installer/PowerToysSetupVNext/generateAllFileComponents.ps1` keeps only
the root copy when WinUI3Apps and root copies are byte-identical, so a
single root-level entry is sufficient.

## Follow-up

After merge to `main`, cherry-pick / merge into `stable` to unblock the
0.100 release pipeline. This is the third (and hopefully final) PR in
the 0.100 release-pipeline unblock sequence, after #48050 (sign
`YamlDotNet.dll`) and #48054 (remove duplicate
`QuickAccent_SelectedLanguage_Greek_Polytonic` resource).

Co-authored-by: Copilot <Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>