web target should allow for importing npm modules

[Pauan]

When using the following code with --target web...

#[wasm_bindgen(module = "foo")]
extern "C" {
    fn foo();
}

...it gives the following error:

error: NPM dependencies have been specified in `package.json` but this is only compatible with the `bundler` and `nodejs` targets

This error is confusing, but more importantly we should allow for npm modules with --target web because:

1. The Rollup plugin uses --target web internally because it cannot use --target bundler. Rollup can handle npm modules perfectly fine, so this should be allowed.

2. There are plans to have web native modules, such as std::kv-storage (which was ultimately removed, but there will be other native modules in the future).

3. By using import maps it is possible to load npm modules in the browser.