Exclude outDir from tsconfig

Consuming output as input is generally discouraged since it adds memory overhead without adding value. As a bonus, this makes it possible to build twice in a row without cleaning in between (though I suspect this file is only for editor support and the actual build happens through webpack).

Microsoft Reviewers: Open in CodeFlow

Disclosure: I don't actually know what this project does - I was just building arbitrary tsconfig projects from GH to validate the TypeScript compiler.

Edit: Just looked through the readme. Cool! If this project is instructional, this is definitely a best practice we'd like to role model.