Refactor extension to support arbitrary shell command runnables
Currently, the extension assumes that all runnables invoke cargo. Arguments are sometimes full CLI arguments, and sometimes arguments passed to a cargo subcommand.
Refactor the extension so that tasks are just a `program` and a list of strings `args`, and rename `CargoTask` to `RustTask` to make it generic.
(This was factored out of #16135 and tidied.)
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