Show a message if a newer version is available

[nobsaibot]

Hi everyone!
And thank you for this awesome project!

I found this feature in the GitHub CLI application and npm.

I think it'll be useful for users who install deno using the installation script.
And this will let them know that a new version is available and it's time to update.

Ready-made crates to do that:

• https://github.com/mgrachev/update-informer
• https://github.com/tarikeshaq/update-notifier

[ducaale]

In case it helps, a similar discussion happened in ducaale/xh#249

• Considering security/performance implications, I'm not a fan of tools that auto-check for updates without the user's consent.
• However, a good compromise might be to only check for updates when deno --version is used (refer to terraform version for prior art).
• The notifier should probably be disabled if deno is installed using a package manager.

[lino-levan]

This was added when #15974 was merged. Should this issue be closed?