Add a slash command for automatically retrieving relevant context (#17972)

* [x] put this slash command behind a feature flag until we release
embedding access to the general population
* [x] choose a name for this slash command and name the rust module to
match

Release Notes:

- N/A

---------

Co-authored-by: Jason <jason@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Jason Mancuso <7891333+jvmncs@users.noreply.github.com>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/semantic_index/src/embedding.rs

@@ -42,14 +42,23 @@ impl Embedding {
         self.0.len()
     }
 
-    pub fn similarity(self, other: &Embedding) -> f32 {
-        debug_assert_eq!(self.0.len(), other.0.len());
-        self.0
+    pub fn similarity(&self, others: &[Embedding]) -> (f32, usize) {
+        debug_assert!(others.iter().all(|other| self.0.len() == other.0.len()));
+        others
             .iter()
-            .copied()
-            .zip(other.0.iter().copied())
-            .map(|(a, b)| a * b)
-            .sum()
+            .enumerate()
+            .map(|(index, other)| {
+                let dot_product: f32 = self
+                    .0
+                    .iter()
+                    .copied()
+                    .zip(other.0.iter().copied())
+                    .map(|(a, b)| a * b)
+                    .sum();
+                (dot_product, index)
+            })
+            .max_by(|a, b| a.0.partial_cmp(&b.0).unwrap_or(std::cmp::Ordering::Equal))
+            .unwrap_or((0.0, 0))
     }
 }