build.rs that builds the frontend too

[trevyn]

Make sure npm install is run if no node_modules and do an npm run build.

[trevyn]

Looks like this is going to be challenging due to rust-lang/cargo#6412

Maybe just have build.rs make sure that frontend/node_modules and frontend/src/turbocharger_generated are present (and up to date??) and if there's any issues, error the build with advice.

Or use a separate build script and have a plain cargo build fail with advice to use the build script.

Or just use npm run build npm run dev npm run watch etc. and have a build.rs that errors out and suggests these if everything is not installed and up to date (e.g. by checking mod times on turbocharger_generated etc.

[trevyn]

Just using npm now.