feat: Turtle.get/set_IsPenUp ( Fixes #45 )

Also updating Forward to respect this setting

Author: James Brundage

Types/Turtle/Forward.ps1

@@ -6,5 +6,9 @@ $Distance = 10
 $x = $Distance * [math]::round([math]::cos($this.Heading * [Math]::PI / 180),15)
 $y = $Distance * [math]::round([math]::sin($this.Heading * [Math]::PI / 180),15)
 $this.Position = $x, $y
-$this.Steps += "l $x $y"
+if ($This.IsPenDown) {
+    $this.Steps += " l $x $y"
+} else {
+    $this.Steps += " m $x $y"
+}
 return $this

Types/Turtle/get_IsPenDown.ps1

@@ -0,0 +1,2 @@
+if ($null -ne $this.'.IsPenDown') { return $this.'.IsPenDown' }
+return $true

Types/Turtle/set_IsPenDown.ps1

@@ -0,0 +1,11 @@
+param(
+[bool]
+$IsDown
+)
+
+$this | 
+    Add-Member -MemberType NoteProperty -Force -Name '.IsPenDown' -Value $IsDown
+    
+if ($VerbosePreference -ne 'SilentlyContinue') {
+    Write-Verbose "Turtle is now $($IsDown ? 'down' : 'up')"
+}