helix: Allow yank without a selection (#35612)

Related https://github.com/zed-industries/zed/issues/4642

Release Notes:
- Helix: without active selection, pressing `y` in helix mode will yank
a single character under cursor.

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/vim/src/test/vim_test_context.rs

@@ -143,6 +143,16 @@ impl VimTestContext {
         })
     }
 
+    pub fn enable_helix(&mut self) {
+        self.cx.update(|_, cx| {
+            SettingsStore::update_global(cx, |store, cx| {
+                store.update_user_settings::<vim_mode_setting::HelixModeSetting>(cx, |s| {
+                    *s = Some(true)
+                });
+            });
+        })
+    }
+
     pub fn mode(&mut self) -> Mode {
         self.update_editor(|editor, _, cx| editor.addon::<VimAddon>().unwrap().entity.read(cx).mode)
     }
@@ -210,6 +220,26 @@ impl VimTestContext {
         assert_eq!(self.mode(), Mode::Normal, "{}", self.assertion_context());
         assert_eq!(self.active_operator(), None, "{}", self.assertion_context());
     }
+
+    pub fn shared_clipboard(&mut self) -> VimClipboard {
+        VimClipboard {
+            editor: self
+                .read_from_clipboard()
+                .map(|item| item.text().unwrap().to_string())
+                .unwrap_or_default(),
+        }
+    }
+}
+
+pub struct VimClipboard {
+    editor: String,
+}
+
+impl VimClipboard {
+    #[track_caller]
+    pub fn assert_eq(&self, expected: &str) {
+        assert_eq!(self.editor, expected);
+    }
 }
 
 impl Deref for VimTestContext {