Add Apache 2.0 LICENSE

[natsukagami]

Same one as in https://github.com/scala/scala3

[natsukagami]

@belamenso @m8nmueller @odersky @He-Pin @SethTisue @2m @felixonmars
Hello gears contributors,
I am mentioning you in this PR because we want to add an open source license to the repo, which will grant rights as written in the LICENSE file to all code in the repository, including yours (which right now are under copyright).
Please leave an indication of your agreement (a comment saying so, a thumbs-up reaction to this message) if you are okay with the Apache 2.0 license (which is the same as Dotty/scala3's) and wish us to proceed with adding the license. If you have questions or other objections, please feel free to do so in this PR.
Thank you!

[He-Pin]

@nicolasstucki I'm totally OK, go ahead.

[belamenso]

Regarding my contributions, I approve.

[natsukagami]

Thanks everyone!