Switch the --browser argument to --web

This commit reverts part of the implementation of [RFC 6]. That RFC
specified that the `--browser` flag was going to be repurposed for the
new "natively loadable as ES module output", but unfortunately the
breakage is far broader than initially expected. It turns out that
`wasm-pack` passes `--browser` by default which means that a change to
break `--browser` would break all historical versions of `wasm-pack`
which is a bit much for now.

To solve this the `--browser` flag is going back to what it represents
on the current released version of `wasm-bindgen` (optimize away some
node.js checks in a few places for bundler-style output) and a new
`--web` flag is being introduced as the new deployment strategy.

[RFC 6]: https://github.com/rustwasm/rfcs/pull/6

Closes #1318

crates/cli/src/bin/wasm-bindgen-test-runner/main.rs

@@ -108,7 +108,7 @@ fn rmain() -> Result<(), Error> {
     let mut b = Bindgen::new();
     b.debug(debug)
         .nodejs(node)?
-        .browser(!node)?
+        .web(!node)?
         .input_module(module, wasm)
         .keep_debug(false)
         .emit_start(false)