Support standalone (non-npm) usage

I'm working in more multi-language monorepos lately, and I'm having a hard time finding a cross-language script runner that's nearly as good or easy to understand as Wireit.

Wireit is already usable in non-npm packages by configuring the package to also be an npm package and using the `npm` command to run scripts, but it would be a bit better if Wireit could be invoked without npm.

I think this could be as simple as supporting a `wireit.json` file describing the scripts and a `wireit` CLI to run them.