add reset to the color palette (#8083)

book/src/themes.md

@@ -70,6 +70,7 @@ ### Color palettes
 
 | Color Name      |
 | ---             |
+| `reset`         |
 | `black`         |
 | `red`           |
 | `green`         |

helix-view/src/theme.rs

@@ -359,6 +359,7 @@ impl Default for ThemePalette {
     fn default() -> Self {
         Self {
             palette: hashmap! {
+                "reset".to_string() => Color::Reset,
                 "black".to_string() => Color::Black,
                 "red".to_string() => Color::Red,
                 "green".to_string() => Color::Green,