theme: horizon-dark fix constructor color (helix-editor#9493)

* theme: add horizon-dark

* fix whitespace color and gutter selected

* taplo fmt

* markup and color tweaks

markup colors
diff colors
better comment and menu colors

* horizon-dark: fix constructor color

makes Some and None to look better and more like the vscode theme

runtime/themes/horizon-dark.toml

@@ -11,6 +11,7 @@ keyword = "purple"
 function = "blue"
 label = "orange"
 type = "orange"
+constructor = "orange"
 namespace = "orange"
 
 # User Interface