Double-width bracket and Flex ATX PSU Module

First of all, this is an awesome project. It works beautifully.

I wanted a way to mount my [Flex ATX PSU](https://www.amazon.com/dp/B08J2NDBWY), so I made a couple of models and thought they might be useful to others.

I modified the [existing bracket](https://github.com/JaredC01/LabStack/blob/main/LabStack%20Mini/STLs/2U%20Mini%204x%20LabStack%20Module%20M3x4x5mm%20Heatset%20Bracket.stl) to hold a "double-width" module:

STL here: https://github.com/adamhl8/share/blob/main/LabStack%20Double-width%20Bracket.stl

And then here's a module that holds a standard Flex ATX PSU:

STL here: https://github.com/adamhl8/share/blob/main/LabStack%20Flex%20ATX%20PSU%20Module.stl

Using this in my RackMate T1:

檢查 LabStack Mini/STLs/2U Mini 4x LabStack Module M3x4x5mm Heatset Bracket.stl 中現有的 STL，然後比較連結的 Double-width Bracket 和 Flex ATX PSU Module 檔案。確認專案是否希望將這些託管在外部的模型加入其 STL 集合；完成的標準是預期檔案已存在，並且可以與現有的 LabStack 模組設計搭配使用。

由索引模型根據 Issue 內容生成。