feat: Add on-typing handler for left angle

Only advertise this feature in the server capabilities when the client
supports SnippetTextEdit.

Close #11398.

Co-authored-by: unexge <unexge@gmail.com>

crates/rust-analyzer/src/config.rs

@@ -1070,6 +1070,10 @@ impl Config {
         }
     }
 
+    pub fn snippet_cap(&self) -> bool {
+        self.experimental("snippetTextEdit")
+    }
+
     pub fn assist(&self) -> AssistConfig {
         AssistConfig {
             snippet_cap: SnippetCap::new(self.experimental("snippetTextEdit")),