Support multi-cursor autocompletion

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
Co-Authored-By: Max Brunsfeld <max@zed.dev>

crates/editor/src/multi_buffer.rs

@@ -1314,6 +1314,12 @@ impl MultiBufferSnapshot {
         }
     }
 
+    pub fn bytes_at<'a, T: ToOffset>(&'a self, position: T) -> impl 'a + Iterator<Item = u8> {
+        self.bytes_in_range(position.to_offset(self)..self.len())
+            .flatten()
+            .copied()
+    }
+
     pub fn buffer_rows<'a>(&'a self, start_row: u32) -> MultiBufferRows<'a> {
         let mut result = MultiBufferRows {
             buffer_row_range: 0..0,