message: document build.rs
diff: +/// This file customizes debug builds for Wrangler
  +///
  +/// 1) sets the debug flag for rustc so Wrangler can feature detect
  +/// https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-cfg
  +/// 2) set the source directory for Wrangler so debug builds know where to
  +/// look for wranglerjs source files
  +/// https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-envvarvalue
  use std::env;
  fn main() {
  