Use IIFE for injected javascript.

[tmpfs]

Otherwise the global name variable is overriden by the last assigned callback and all function invocations target the last defined callback.

This fixes a bug whereby using multiple callbacks when calling add_window like this:

app.add_window(attrs, Some(vec![ipc, log_error, log_info, log_warn]))?;

Without this fix all javascript function invocations call the Callback named log_warn (the last one defined); after applying this fix each Callback should be invoked as expected.

[lucasfernog]

We should have added a change file on this PR, totally forgot that we have covector now :|