Dispose extension files in EditorAppExtended

[vrama628]

Fixes #679

There was a bug where disposing and then reinitializing an EditorAppExtended where an extension has filesOrContents would result in an exception being thrown due to not disposing these files from the last time the extension was registered. This fixes that by disposing of the results of registerFileUrl in the disposeApp method.

I added a test to exercise this; the test introduced in the first commit exhibits the Error: file 'extension-file://undefined.undefined/' already exists error, and that error goes away in the second commit. Unfortunately, when I run this test suite, I also get a bunch of "Array buffer allocation failed" errors that seem to come from somewhere in VSCode's services. I tried using the updateExtendedAppPrototyp test helper to fix this, but that seems to remove so much functionality that the original "file already exists" error no longer happens in the first place. Open to suggestions on how to fix the test, or I can just remove the test so we can ship the fix.

[kaisalmen]

@vrama628 thank you for your contribution. Please move the test to a different Draft PR. I don't have a solution for that and need to investigate further. updateExtendedAppPrototyp is a hack/workaround for the tests.

[vrama628]

@kaisalmen Thanks, I've moved the test into a separate draft PR #688 and updated to import { DisposableStore } from 'vscode/monaco'.

[kaisalmen]

LGTM. Thank you.