🩹 [Patch]: Consolidate Install-PSModuleHelpers into Install-PSModule (#397)

The `Install-PSModuleHelpers` action step has been consolidated under
the unified `Install-PSModule` name across all workflow files and
composite actions. Obsolete test fixture files that are no longer needed
by the test repositories have also been removed.

- Fixes #347

## Changed: Install step unified under `Install-PSModule`

The internal step name `Install-PSModuleHelpers` is renamed to
`Install-PSModule` in all reusable workflows and composite actions. This
affects:

- `.github/workflows/AfterAll-ModuleLocal.yml`
- `.github/workflows/BeforeAll-ModuleLocal.yml`
- `.github/workflows/Build-Site.yml`
- `.github/workflows/Test-ModuleLocal.yml`
- `.github/actions/Build-PSModule/action.yml`
- `.github/actions/Document-PSModule/action.yml`

There is no change to the behavior or inputs/outputs of these workflows
— this is a naming consolidation only.

## Changed: Obsolete test fixtures removed

Unused source files, icons, assemblies, and configuration fixtures from
the test repositories (`srcTestRepo` and `srcWithManifestTestRepo`) have
been removed, reducing noise in the repo and simplifying the test
structure.

## Technical Details

- All step references `uses:
./_wf/.github/actions/Install-PSModuleHelpers` replaced with `uses:
./_wf/.github/actions/Install-PSModule`.
- Deleted: `src/assemblies/LsonLib.dll`, `src/classes/`, `src/data/`,
`src/finally.ps1`, `icon/`, `README.md`, `mkdocs.yml`, and several other
fixtures from `srcTestRepo`.
- No consumer-visible API or behavior changes.

Author: MariusStorhaug

.github/actions/Build-PSModule/action.yml

@@ -28,8 +28,8 @@ inputs:
 runs:
   using: composite
   steps:
-    - name: Install-PSModuleHelpers
-      uses: ./_wf/.github/actions/Install-PSModuleHelpers
+    - name: Install-PSModule
+      uses: ./_wf/.github/actions/Install-PSModule
 
     - name: Run Build-PSModule
       shell: pwsh