Fixes #218626

[swordensen]

This is to resolve https://github.com/microsoft/vscode/issues/218626

Basically, it prevents windows from popping up when dragging files over to extensions from the OS's file explorer.

You can test this functionality by performing the following steps:

1. clone repo https://github.com/maxbublik/vscode-customeditor-drop
2. press f5 to open demo repo as an extension
3. drag and drop any file into the extension editor while holding shift
4. notice that a new window did not appear