Add PowerShell / Pester workflow

ci/powershell.yml

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+# This workflow will run PowerShell tests on Windows, Linux and macOS
+name: PowerShell
+
+on:
+  push:
+    branches: [ $default-branch ]
+  pull_request:
+    branches: [ $default-branch ]
+
+jobs:
+  build:
+    name: Module imports on all platforms
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macOS-latest]
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Perform one Pester test to ensure files or folders are in the current directory
+      shell: pwsh
+      run: Get-ChildItem | Should -Not -BeNullOrEmpty
+    - name: Perform two Pester tests to ensure expected results
+      shell: pwsh
+      run: |
+        Get-ChildItem | Select-Object -ExpandProperty Name -First 1 | Should -Be '.github'
+        Get-Module Pester -ListAvailable | Measure-Object | Select -ExpandProperty Count | Should -BeGreaterThan 0
+    # Want to setup more advanced tests with Descriptions, Contexts and more? Use a ps1 script in your respository instead.
+    - name: Run a script in the tests directory that has advanced Pester testing 
+      shell: pwsh
+      run: ./tests/pester.ps1

ci/properties/powershell.properties.json

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+{
+    "name": "PowerShell",
+    "description": "Test PowerShell commands on multiple OS platforms.",
+    "iconName": "powershell",
+    "categories": ["PowerShell"]
+}