Fix getDocument function to not return a Log document

[bpringe]

This is located in src/utilities.ts. Currently if no editors are open, but the bottom pane is open with, for example, the Output -> Tasks pane showing, then the document returned from getDocument has a .uri.fsPath of tasks. This function should only return an editor whose document is an actual file.

This may be best accomplished by filtering visible text editors by languageId. (It's Log when the document.uri.fsPath is tasks.)