fix: PowerShell.Create(Runspace) has no such overload on PS 5.1

[System.Management.Automation.PowerShell]::Create(Runspace) doesn't
exist as an overload in Windows PowerShell 5.1's System.Management.
Automation (v3.0.0.0, GAC-loaded) -- only Create(), Create(RunspaceMode),
and Create(InitialSessionState) are available there. Calling it with a
Runspace argument throws:

  MethodException: Cannot find an overload for "Create" and the
  argument count: "1".

Split into Create() followed by assigning the writable Runspace
property instead, which works on both PS 5.1 and 7+.

Fixes both call sites (New-KoanRunspace and Invoke-Koan) plus the
test file's own use of the same broken pattern.

Verified locally: New-KoanRunspace.Tests.ps1 + Invoke-Koan.Tests.ps1
pass under both powershell.exe 5.1 and pwsh 7.

Author: HeyItsGilbert

PSKoans/Private/Invoke-Koan.ps1

@@ -61,7 +61,8 @@
                 }
             }
 
-            $ps = [powershell]::Create($script:KoanRunspace)
+            $ps = [powershell]::Create()
+            $ps.Runspace = $script:KoanRunspace
             $ps.AddScript($Script, <# useLocalScope: #> $true) > $null
 
             $ps.AddParameter('Params', $ParameterSplat) > $null

PSKoans/Private/New-KoanRunspace.ps1

@@ -22,7 +22,8 @@ function New-KoanRunspace {
     $runspace = [runspacefactory]::CreateRunspace()
     $runspace.Open()
     $runspace.Name = 'PSKoans.KoanRunspace'
-    $ps = [powershell]::Create($runspace)
+    $ps = [powershell]::Create()
+    $ps.Runspace = $runspace
 
     try {
         $script = {

Tests/Functions/Private/New-KoanRunspace.Tests.ps1

@@ -44,7 +44,8 @@ Describe 'New-KoanRunspace' {
 
     It 'preloads the runspace with PSKoans' {
         $runspace = InModuleScope 'PSKoans' { New-KoanRunspace }
-        $ps = [powershell]::Create($runspace)
+        $ps = [powershell]::Create()
+        $ps.Runspace = $runspace
 
         try {
             $ps.AddCommand('Get-Module').AddParameter('Name', 'PSKoans') > $null