Extensions: support for custom item in Tasks: Configure Task Runner

[joshtynjala]

I'm working on a new extension for a language, and I'd love the ability to add an item to the list shown when you create "Tasks: Configure Task Runner" command. If that's possible now, I just can't seem to find it in the documentation.

See the following quick-and-dirty mockup:

We can add custom items to "Debug: open launch.json" using contributes:debuggers in the extension's package.json, so there is precedence for this sort of thing.

The reason I would like this API is so that I can create a template for the tasks.json file that runs a specific executable, similar to how "TypeScript - tsconfig.json" runs "tsc" or "MSBuild" runs "msbuild". That's easier for users than having to choose "Other" and enter the details manually.

Thank you!