Remove cx from ThemeSettings (#38836)

Before this change the active theme and icon theme were retrofitted onto
the ThemeSettings.

Now they're in their own new global (GlobalTheme::theme(cx) and
GlobalTheme::icon_theme(cx))

This lets us remove cx from the settings traits, and tidy up a few other
things along the way.

Release Notes:

- N/A

crates/vim/src/vim.rs

@@ -1913,7 +1913,7 @@ impl From<settings::ModeContent> for Mode {
 }
 
 impl Settings for VimSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let vim = content.vim.clone().unwrap();
         Self {
             default_mode: vim.default_mode.unwrap().into(),