Custom editors should not allow to make edits after extension host restart

This is a follow up from https://github.com/microsoft/vscode/issues/180514

When the EH restarts with an opened custom editor, that editor continues to be editable but does not report any dirty indication and does not allow to be saved because the old EH it was talking to is gone.

I think a custom editor should detect that it lost the connection to its extension on the EH side and then do something, such as:

• turn readonly to prevent further edits
• show an indication to the user what happened

//cc @Tyriar