Delegate TCL folding to server, fix bugs, and enhance stability

- Move TCL folding range computation from client to language server, avoiding duplicate regions.
- Serialize language server restarts to prevent multiple server instances running concurrently.
- Pin server-side Python dependencies to specific minor versions for improved stability and predictability.
- Make debug server connection non-fatal, allowing the language server to start even if the debugger isn't attached.
- Optimize semantic token generation by caching Tclint plugin commands.

server/src/_debug_server.py

@@ -27,7 +27,17 @@ if debugger_path:
 
     # 5678 is the default port, If you need to change it update it here
     # and in launch.json.
-    debugpy.connect(5678)
+    # Connecting requires the "Python debug server" listener (launch.json) to be
+    # up first. If it isn't (e.g. wrong launch config was used), don't crash the
+    # whole language server - just continue running without the debugger attached.
+    try:
+        debugpy.connect(5678)
+    except (ConnectionRefusedError, OSError) as exc:
+        print(
+            f"debugpy: could not connect to debug adapter on port 5678 "
+            f"({exc}); continuing without debugging.",
+            file=sys.stderr,
+        )
 
     # This will ensure that execution is paused as soon as the debugger
     # connects to VS Code. If you don't want to pause here comment this