Generating build/debug assets should not overwrite existing launch and tasks

Currently when generating build/debug assets for .NET Core projects, anything in launch.json and tasks.json will be overwritten. This can cause data loss and user confusion. Instead, the relevant launch and tasks should be added if missing, and if already present, prompted for overwriting.

The vscode-docker extension does this by checking the name of the launch/task. If another launch/task of the same name we're about to create exists, we will prompt if they want to overwrite.