theme: add doom-flate

[Arsenic-ATG]

This is a theme heavily inspired by VSCode's Flate theme, I created it as I wasn't able to find this theme implemented for emacs (or literally any other editor) anywhere on the internet.

This is my first time creating any theme and contributing to this repository, so please let me know if there is something I am missing here, or something you would like to change.

I have been using this theme for my emacs for a few days now and haven't checked how it looks in all modes, but am ready to maintain this theme and fix stuff if anything goes wrong while using any mode in future.

Fixes #0000
References #0000
Replaces #0000

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• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to the git conventions.
• My changes are visual; I've included before and after screenshots.
• I am blindly checking these off.
• Any relevant issues or PRs have been linked to.
• This a draft PR; I need more time to finish it.

screenshots

[gagbo]

There's a duplicate "Author" line in the theme file, but otherwise it's good to go