feat: add everforest themes

[Echinoidea]

This PR adds three themes: doom-everforest-hard, doom-everforest-medium, and doom-everforest-soft.

I have not found any Everforest themes that work well with Doom emacs, particularly with packages like vterm.

There was one issue some time ago that requested this, but nothing came of it. #808

These theme files are based on doom-miramare-theme.el for no particular reason. The colors were taken from the official Everforest repository [https://github.com/sainnhe/everforest] and is credited in the file headers.

Everforest Soft

Everforest Medium

Everforest Hard

Fix: #0000 Ref: #0000 Close: #0000

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

[ImYuugen]

Hello, I am interested with working on this, if you are still planning to merge this PR. I noticed that it is not marked as draft, but there are some things missing

If I can't contribute directly for some reason, here are some remarks:

• Shouldn't the new themes be in the themes/ directory ?
• The doom-themes.el file should probably document the new themes in the big comment block
• The README as well

[Echinoidea]

Hello, I am interested with working on this, if you are still planning to merge this PR. I noticed that it is not marked as draft, but there are some things missing

If I can't contribute directly for some reason, here are some remarks:

* Shouldn't the new themes be in the `themes/` directory ?

* The `doom-themes.el` file should probably document the new themes in the big comment block

* The README as well

Yeah you're right. I forgot to put this in the directory and update the other files. I just made the themes. If you want to do the things you mentioned, that would be welcomed. If not, I'll get around to it soon when I'm free.