Split Interactive into Interactive and Toggleable (#2628)

This is a part of the intensity driven theme rewrite. 

It introduces the `toggle` and `interactive` helper functions to build
Toggle<T> and Interactive<T> styles for interactive elements in the
theme.

This PR also removes the `theme_testbench` crate and related actions.

Huge thanks to @osiewicz and @mikayla-maki for pushing this forward 🙏🏽

Release Notes:

- Updated the style of many interactive elements.

crates/language_selector/src/active_buffer_language.rs

@@ -55,7 +55,7 @@ impl View for ActiveBufferLanguage {
 
             MouseEventHandler::<Self, Self>::new(0, cx, |state, cx| {
                 let theme = &theme::current(cx).workspace.status_bar;
-                let style = theme.active_language.style_for(state, false);
+                let style = theme.active_language.style_for(state);
                 Label::new(active_language_text, style.text.clone())
                     .contained()
                     .with_style(style.container)

crates/language_selector/src/language_selector.rs

@@ -180,7 +180,7 @@ impl PickerDelegate for LanguageSelectorDelegate {
     ) -> AnyElement<Picker<Self>> {
         let theme = theme::current(cx);
         let mat = &self.matches[ix];
-        let style = theme.picker.item.style_for(mouse_state, selected);
+        let style = theme.picker.item.in_state(selected).style_for(mouse_state);
         let buffer_language_name = self.buffer.read(cx).language().map(|l| l.name());
         let mut label = mat.string.clone();
         if buffer_language_name.as_deref() == Some(mat.string.as_str()) {