Added ferra theme

[casperstorm]

This PR adds my theme ferra. It's a original theme I created a few years ago. A small description for the theme:

Ferra is a visually appealing theme that is inspired by the vibrant colors of Kanagawa and the warm tones of Gruvbox. This theme features a rich palette of earthy browns and greens, accented by subtle pops of blush, coral, and rose. The result is a harmonious blend of colors that is both inviting and easy on the eyes.

Running cargo xtask themelint ferra shows no current linting errors:

➜  helix git:(theme/ferra) ✗ cargo xtask themelint ferra
   Finished dev [unoptimized + debuginfo] target(s) in 0.13s
   Running `target/debug/xtask themelint ferra`

Let me know what you think, and let me know if you find any errors with it which I might have missed while porting it. I have mostly tested this with Rust.

[pickfire]

Thanks, can you please add it to https://github.com/helix-editor/helix/wiki/Themes too?

[FabianHummel]

What is the name of the wallpaper being used in the screenshot? I would love to know because it fits so well with the theme =)

[casperstorm]

What is the name of the wallpaper being used in the screenshot? I would love to know because it fits so well with the theme =)