feat: Posh.Host.UI.Box.MiddleVerticalWall ( Fixes #375 )

docs/Posh/Host/UI/Box/get_MiddleVerticalWall.md

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+Posh.Host.UI.Box.get_MiddleVerticalWall()
+-----------------------------------------
+
+### Synopsis
+Gets a box's middle vertical wall character
+
+---
+
+### Description
+
+Gets the middle vertical 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.
+
+---