Allow disabling background check for updates

[orhun]

💡 Feature description

Hey, I'm maintaining the wasm-pack package for Arch Linux and I would like to have a way of disabling the background check for updates. This has the following advantages for distro packages:

• Ensures all users have the same version (avoiding compatibility issues and maintaining uniformity)
• Less resource usage (no API call to crates.io)

💻 Basic example

We can simply add a feature flag as follows:

[features]
default = ["check-for-updates"]

Then we can annotate the related functions with #[cfg(feature = "check-for-updates")]. The users who would like to disable this will have to build wasm-pack with --no-default-features flag.

I'm happy to raise a PR if you think this is applicable. 🐻