feat(tazjin/rlox): Bootstrap program

This is going to be the first of two interpreters from "Crafting
Interpreters".

Change-Id: I354ddd2357444648d0245f35d92176dd176525d8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2142
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/main.rs

@@ -0,0 +1,23 @@
+use std::process;
+use std::env;
+
+fn run_file(_file: &str) {
+    unimplemented!("no file support yet")
+}
+
+fn run_prompt() {
+    unimplemented!("no prompt support yet")
+}
+
+fn main() {
+    let mut args = env::args();
+
+    if args.len() > 1 {
+        println!("Usage: rlox [script]");
+        process::exit(1);
+    } else if let Some(file) = args.next() {
+        run_file(&file);
+    } else {
+        run_prompt();
+    }
+}