theme: add doom-gruvbox-material (a second PR)

[romanZupancic]

Hello!

This PR adds Gruvbox Material theme (from https://github.com/sainnhe/gruvbox-material-vscode).

There are several commits, some of which are from PR #714 which I've merged into the latest master and built upon.

• Also, that PR mentions looking into using the VSCode version of the theme instead of the nvim version due to higher color fidelity. I've made those changes here.
• Most of the changes I've made are to more directly define and use the Gruvbox material colours from the original palette.
• I tried to follow the commit format for the latest commit. Let me know if something needs to change (or if these commits should be squashed together).

Additional changes were made to the colours for secondary-selection (and its markdown equivalent) for slightly more pleasurable usage

Thanks!

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(relevant) Checklist:

• [ X ] I searched the issue tracker and this hasn't been PRed before.
  • Issue has been PR'd (theme: add doom-gruvbox-material #714) but that seems very stale (> 3years, missing literally hundreds of upstream commits)
• [ X ] My changes are visual; I've included before and after screenshots.
  No before screenshots, but here is an after one:
  
  The colour changes for secondary-selection is the slight shift in background color you see in the code block under 'Completion Engine'

Close: #714