🩹 [Patch]: Add $IsWindows variable in the root module (#52)

## Description

- Add `$IsWindows` variable in the root module.

## Type of change

<!-- Use the check-boxes [x] on the options that are relevant. -->

- [ ] 📖 [Docs]
- [ ] 🪲 [Fix]
- [x] 🩹 [Patch]
- [ ] ⚠️ [Security fix]
- [ ] 🚀 [Feature]
- [ ] 🌟 [Breaking change]

## Checklist

<!-- Use the check-boxes [x] on the options that are relevant. -->

- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas

scripts/helpers/Build/Build-PSModuleRootModule.ps1

@@ -134,6 +134,20 @@ param()
     }
     #endregion - Module header
 
+    #region - $IsWindows
+    Write-Verbose "Adding variable for `$IsWindows for compatibility with Windows PowerShell."
+    Add-Content -Path $rootModuleFile -Force -Value @'
+
+if ($PSVersionTable.PSVersion -lt '6.0') {
+    [Diagnostics.CodeAnalysis.SuppressMessageAttribute(
+        'PSAvoidAssignmentToAutomaticVariable', '', Justification = 'Compatibility with PowerShell 6.0 and newer.'
+    )]
+    $IsWindows = [System.Environment]::OSVersion.Platform -eq 'Win32NT'
+}
+
+'@
+    #endregion - $IsWindows
+
     #region - Module post-header
     Add-Content -Path $rootModuleFile -Force -Value @"
 `$scriptName = '$ModuleName'