lsp: Backport language_id patch from pygls

While the `language_id` patch is still unreleased and that we're still
supporting Python 3.6 (for now) this backports the functionality into
esbonio directly.

lib/esbonio/esbonio/lsp/__init__.py

@@ -152,7 +152,11 @@ def on_shutdown(ls: RstLanguageServer, *args):
 
     @server.feature(TEXT_DOCUMENT_DID_OPEN)
     def on_open(ls: RstLanguageServer, params: DidOpenTextDocumentParams):
-        pass
+
+        # TODO: Delete me when we've dropped Python 3.6 and a pygls release that
+        # remembers the language id is available.
+        doc = ls.workspace.get_document(params.text_document.uri)
+        doc.language_id = params.text_document.language_id  # type: ignore
 
     @server.feature(TEXT_DOCUMENT_DID_CHANGE)
     def on_change(ls: RstLanguageServer, params: DidChangeTextDocumentParams):