add file icon for `.djinni` files

[jothepro]

As a user I want to see a special file icon for .djinni files in the file-explorer.

Currently .djinni files have a simle text-file icon:

I would like to have a special icon like in the intellij-plugin:

[a4z]

The way to implement it is probably described here:
microsoft/vscode#14662

[jothepro]

Nice, good news that it is now supported to have custom icons in vscode! 😊