This consolidates all of the non-template .gitignore rules into the top level .gitignore, to ignore common things more broadly, with less maintenance needed for the .gitignore files. Does not touch the templates, so that they still produce needed .gitignores as part of flutter create.
We now use a different approach for ios projects where the developer controls
the Xcode project file. This patch removes the old ".generated" approach in
favor of the new approach.
