Documents are ignored during initial indexing when already opened in an editor during VSCode startup

[sailingKieler]

Explanation:

• LS initialized event causes the workspace manager beginning its initially scan for files, the subsequent start of the initially document build is guarded by https://github.com/langium/langium/blob/5ab46cf82543c75c26750a38cca4b00323fd7b2a/packages/langium/src/workspace/workspace-manager.ts#L79, which here causes a cancelation because of an immediately following
• onDidOpenTextDocument event for trafficlight.statemachine, which (after canceling previously scheduled build activities) causes the deletion of the previously created langiumDocument and scheduling of a subsequent (still initial) build of that document via https://github.com/langium/langium/blob/5ab46cf82543c75c26750a38cca4b00323fd7b2a/packages/langium/src/workspace/document-builder.ts#L86-L92 that in turn gets canceled at the cancelation check, as a second
• onDidOpenTextDocument event for trafficlightExtended.statemachine comes in that (again after canceling previously scheduled build activities) triggers the deletion of the previously created langiumDocument and scheduling of a subsequent (still initial) build of that document. That build will eventually be executed with the document list of [ trafficlightExtended.statemachine ].
  As a consequence, trafficlight.statemachine has never been built, especially never indexed.
  Thus, error markers on that document in the IDE are not up-to-date, and other documents suffer linking errors.