<!-- Enter a brief description/summary of your PR here. What does it fix/what does it change/how was it tested (even manually, if necessary)? -->
## Summary of the Pull Request
This introduces a setting to both Profiles and ColorSchemes called <code>selectionBackground</code> that allows you to change the selection background color to what's specified. If <code>selectionBackground</code> isn't set in either the profile or color scheme, it'll default to what it was before - white.
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## PR Checklist
* [x] Closes#3326
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Tests added/passed
* [x] Requires documentation to be updated
<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
- Added selectionBackground to existing profile and colorscheme tests.
- Verified that the color does change to what I expect it to be when I add "selectionBackground" to either/both a profile and a color scheme.
<hr>
* adding selectionBackground to ColorScheme and TerminalSettings
* Changing PaintSelection inside the renderers to take a SelectionBackground COLORREF
* changes to conhost and terminal renderdata, and to terminal settings and core
* IT WORKS
* modification of unit tests, json schemas, reordering of functions
* more movement
* changed a couple of unit tests to add selectionBackground, added the setting to schemas, also added the optional setting to profiles
* default selection background should be slightly offwhite like the default foreground is
* reverting changes to .sln
* cleaning up
* adding comment
* oops
* added clangformat to my vs hehe
* moving selectionBackground to IControlSettings and removing from ICoreSettings
* trying to figure out why the WHOLE FILE LOOKS LIKE ITS CHANGED
* here it goes again
* pls
* adding default foreground as the default for selection background in dx
a404778271