Based on the book Crafting Interpreters - http://craftinginterpreters.com/
To build the desktop version, run
$ zig build-exe src/main.zig
To run the repl, run it with no arguments
$ ./main
To run a script, pass the filename as the first parameter
$ ./main example/script.zag
To build the web version, run
$ zig build-exe src/main.zig -target wasm32-freestanding --release-small