json-extension: use an unique token for progress example

The spec [1] and pygls implementation [2] needs the token to be unique.

By using an hardcoded token like this, the progress works only one time,
the second time there's an error that token is already registered.

1: https://microsoft.github.io/language-server-protocol/specifications/specification-current/#serverInitiatedProgress

2: https://github.com/openlawlibrary/pygls//blob/ee17487e4f40f971e7ec6f7711fa8334c5b7b127/pygls/progress.py#L28

examples/json-extension/server/server.py

@@ -197,7 +197,7 @@ def semantic_tokens(ls: JsonLanguageServer, params: SemanticTokensParams):
 @json_server.command(JsonLanguageServer.CMD_PROGRESS)
 async def progress(ls: JsonLanguageServer, *args):
     """Create and start the progress on the client."""
-    token = 'token'
+    token = str(uuid.uuid4())
     # Create
     await ls.progress.create_async(token)
     # Begin