feat: Posh.Host.UI.Box.MiddleWall ( Fixes #371 )

Types/Posh.Host.UI.Box/get_MiddleWall.ps1

@@ -0,0 +1,29 @@
+<#
+.SYNOPSIS
+    Gets a box's middle wall character
+.DESCRIPTION
+    Gets the middle wall in a box.
+.NOTES
+    If the box only has one line, and it is less than 5 characters, the second character will be considered the top wall.
+    
+    If it only has one character, it will be considered the middle wall.
+
+    Otherwise, the character directly left of the middle will be considered the middle wall.
+#>
+$theseLines = @($this -split '[\r\n]+' -ne '')
+
+if (-not $theseLines) { return }
+$theLine = $theseLines[[Math]::Floor($theseLines.Length / 2)]
+if ($theLine.Length -lt 5) {
+    # If there is only one line, it is between two and five characters    
+    if ($theLine.Length -ge 2) {
+        # pick the second character as the top wall
+        $theLine.Substring(1,1)
+    } else {
+        # (if there was only one character, it's the top wall)
+        $theLine.Substring(0,1)
+    }    
+} else {
+    # Otherwise, pick one left of the middle in the line
+    $theLine.Substring(([Math]::Floor($theLine.Length / 2) - 1), 1)
+}