mach: sort out CLI arguments / program options

[emidoots]

• If one follows the getting started guide, it's not possible to easily modify the build.zig to pass arguments from zig build run -- foobar to your program.
  • i.e. you can't call .addArgs because getInstallStep returns a Step not a RunStep.
  • We cannot return a RunStep, since for WASM targets the step runs wasmserve so you can open in your browser.
  • We may need to expose a separate function like runNative (squirl's suggestion), or come up with a different solution.
  • WASM (browser) targets, and likely also other targets (mobile, VR, etc.) do not really have a concept of CLI flags. We may need to come up with some other form of simple "options" one can pass to their program that works in all these situations nicely.
• Whatever we come up with, it should have a documentation section on the new machengine.org website: "adding CLI arguments" or similar.