Improve the yaml outline to show the key value if it's a simple string (#35562)

Before:

<img width="753" height="890" alt="Screenshot 2025-08-03 at 8 58 16 PM"
src="https://github.com/user-attachments/assets/a3816acd-66b2-4042-8181-fbc15881bd4b"
/>

After:

<img width="648" height="634" alt="Screenshot 2025-08-03 at 8 59 30 PM"
src="https://github.com/user-attachments/assets/0ab35f3e-36d6-42b5-bb5f-09431985878e"
/>

Release Notes:

- Improved the yaml outline to include the key's value if it's a simple
string.

crates/languages/src/yaml/outline.scm

@@ -1 +1,9 @@
-(block_mapping_pair key: (flow_node (plain_scalar (string_scalar) @name))) @item
+(block_mapping_pair
+    key:
+        (flow_node
+            (plain_scalar
+                (string_scalar) @name))
+    value:
+        (flow_node
+            (plain_scalar
+                (string_scalar) @context))?) @item