Automatically add file file top module tree or display option to do so.

[FelixBrakel]

Currently when you create a new .rs file in a project it will give you the following notification:

Which is nice. However I thought it would be nice for this notification to include a button to automatically add the following statement to the corresponding mod.rs file (Or whatever the module file is when using the rust 2018 syntax).

mod scene

Such that the file scene.rs is included in the module tree.

Or automatically write this statement when a new .rs file is created. I can't really see a scenario where you wouldn't want to add a file to the module tree so I would personally prefer the latter option but I wouldn't presume to know other people's workflow so the former option might be better because it's less intrusive.