Fix panic when doing various cursor movements with a pending mouse selection (#13016)

This fixes a panic in the `SelectionsCollection::first_anchor` when
there was a pending mouse selection and no other selections. Until
recently, this method was only used in vim mode, but as of
53b0720d54,
it's also used in the normal `move_up` and `move_down` actions.

So until recently, the panic that this fixes could only happen in vim
mode.

Release Notes:

- Fixed a crash that could happen when using certain cursor-motion
bindings with a pending mouse selection.

crates/editor/src/selections_collection.rs

@@ -256,7 +256,10 @@ impl SelectionsCollection {
     }
 
     pub fn first_anchor(&self) -> Selection<Anchor> {
-        self.disjoint[0].clone()
+        self.pending
+            .as_ref()
+            .map(|pending| pending.selection.clone())
+            .unwrap_or_else(|| self.disjoint.first().cloned().unwrap())
     }
 
     pub fn first<D: TextDimension + Ord + Sub<D, Output = D>>(