Add Halcyon theme

[Burloe]

I made a theme called Halcyon that I'd like to ask to be included in the list: https://github.com/Burloe/Halcyon-AsepriteTheme
Apologies if this is the wrong method to ask!

README.md with Halcyon added at the bottom.

<a href="https://github.com/Burloe/Halcyon-AsepriteTheme">Halcyon for Aseprite <br>
<img src="https://raw.githubusercontent.com/Burloe/Halcyon-AsepriteTheme/main/showcase/Showcase1080.png" width="400px" /><a>

[Burloe]

Made a PR #28 to add this