Allow users to set the inactiveRegion color

[bobbrow]

From PR #2061

Add an inactiveRegionColor setting that allows users to set the region color. It should work alongside the opacity setting. If not set, we leave the color alone. If set, we change the color of the inactive regions. VSCode accepts ThemeColors for the decoration, so those could potentially be allowed too.

[bobbrow]

Found a duplicate issue: #1620. Need to close both when this goes in.