Add Halcyon theme (#756)

https://github.com/hichemfantar/halcyon-zed.git

closes #82


![image](https://github.com/zed-industries/extensions/assets/34947993/dfc5df5c-6f28-4e21-9c2b-debc190dd3bf)

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

.gitmodules

@@ -186,6 +186,10 @@
 	path = extensions/gruber-darker
 	url = https://github.com/th0jensen/gruber-darker.zed.git
 
+[submodule "extensions/halcyon"]
+	path = extensions/halcyon
+	url = https://github.com/hichemfantar/halcyon-zed.git
+
 [submodule "extensions/horizon"]
 	path = extensions/horizon
 	url = https://github.com/ayn2op/zed-horizon.git

extensions.toml

@@ -242,6 +242,10 @@ version = "0.0.1"
 submodule = "extensions/gruber-darker"
 version = "0.0.2"
 
+[halcyon]
+submodule = "extensions/halcyon"
+version = "0.0.1"
+
 [haskell]
 submodule = "extensions/zed"
 path = "extensions/haskell"