Fix minimum widths for Shrink and Expand

[dcousens]

Description

I went to use the TwoPane layout while trying out xmonad, and immediately ran into the problem that Shrink and Expand were shrinking my windows into negative width, which broke alacritty.

To investigate the problem, I started reading the Layout.hs from xmonad/xmonad, and noticed that the Tall layout applies a min and max to the respective widths:

• https://github.com/xmonad/xmonad/blob/master/src/XMonad/Layout.hs#L77-L78

Applying this same approach to Shrink and Expand in the TwoPane layouts (tested in my own xmonad.hs) resolves this problem for me, but I am a beginner to xmonad and may be misunderstanding something. 💙

Checklist

• I've read CONTRIBUTING.md

• I've considered how to best test these changes (property, unit,
  manually, ...) and concluded: writing tests is probably beyond my spare time for now

• I updated the CHANGES.md file

[slotThe]

LGTM 👍

[slotThe]

Thank you 🎉