Show a playground version or commit hash somewhere

[tgross35]

Just for the sake of reporting & chasing bugs and behavior differences, it would be nice to be able to record the playground version.

Something simply like this would do the trick:

[shepmaster]

Seems reasonable. Will want a version for the frontend (JS) and the backend (Rust) code. I've used vergen for the Rust side before; I'm sure there's a straight-forward JS (maybe specifically via Webpack) solution as well.