Automatically start `main` on import.

[daxpedda]

Motivation

Especially after #1843, I think it would make a lot of sense that on importing the js file, it would immediately start init and therefore execute main.

This would solve a couple of issues:

• Less boilerplate: <script src="wasm.js" type="module"></script> would be enough.
  • No need to call init.
  • No need to preload the js file (the wasm still needs to be preloaded).
• Can be directly used with Worker, because it starts itself.
• It should only start itself when main or wasm_bindgen(start) is used in rust, which should be pretty intuitive.
• Behaving like other output modes, bundler and nodejs already do that.

Proposed Solution

Call init after its definition if needs_manual_start is true in cli-support.
Additionally we should add a flag to disable this functionality.

Alternatives

• Potentially we could always call init to start loading the wasm file, even if there is no start function.
• Hide this functionality behind a flag instead of making it default and disabling it by a flag.

Additional Context

I am afraid there was some reason that this functionality wasn't implemented already, I just can't figure out why.

EDIT: Just started to implement this when I found the other output modes already do that, only no-modules and web don't.

[daxpedda]

According to @alexcrichton's recommendation (#1940 (comment)) we should wait until ES module integration is out.