Prompt library updates (#11988)

Restructure prompts & the prompt library.

- Prompts are now written in markdown
- The prompt manager has a picker and editable prompts
- Saving isn't wired up yet
- This also removes the "Insert active prompt" button as this concept doesn't exist anymore, and will be replaced with slash commands.

I didn't staff flag this, but if you do play around with it expect it to still be pretty rough.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <1789+nathansobo@users.noreply.github.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/gpui/src/util.rs

@@ -41,6 +41,25 @@ pub trait FluentBuilder {
         })
     }
 
+    /// Conditionally unwrap and modify self with one closure if the given option is Some, or another if it is None.
+    fn when_some_else<T>(
+        self,
+        option: Option<T>,
+        then: impl FnOnce(Self, T) -> Self,
+        otherwise: impl FnOnce(Self) -> Self,
+    ) -> Self
+    where
+        Self: Sized,
+    {
+        self.map(|this| {
+            if let Some(value) = option {
+                then(this, value)
+            } else {
+                otherwise(this)
+            }
+        })
+    }
+
     /// Conditionally modify self with one closure or another
     fn when_else(
         self,