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):