Relicense under dual MIT/Apache-2.0

[robo9k]

In the spirit of robo9k/rust-magic-sys#8 this repo here should be dual licensed under both the MIT and the Apache-2.0 licenses.

Previous license related issues: #1 #3 #12 #25 #41 #42 #44

Currently the license is only MIT, not Apache-2.0

Requirement for a relicense like this in future versions is that all contributors of the current files agree with the change.

However I went through git blame of all the files currently present in main (dc3dbc5) and determined that I, @robo9k , am the author of all files or non-whitespace / non-trivial lines in text files.
As such while IANAL I believe my agreement is sufficient for this license change.

Documentation for contributors should state that future contributions will be dual-licensed as described above.

[robo9k]

Some NOTEs on licensing:

• There's a difference between the source form (i.e. this Git repo here) and the distribution form (i.e. .crate files on crates.io).
  I believe it's fine to have e.g. License of Rust logo test file #12 or [dev-dependencies] as long as users of the magic Rust crate don't need to concern themselves with such details.
• [dependencies] used by the distribution form should be MIT OR Apache-2.0 as well.
• Since this is a binding crate, documentation is partially copied from file / libmagic which has a custom, BSD-ish license.
  I think this is not an issue, but the documentation could be replaced / improved anyways.

[robo9k]

Given that this adds another license choice, it should be a semver compatible change.
Either way, people should automatically use the license key from Cargo.toml and not just eyeball a README or such once.

[robo9k]

License is also part of the OpenSSF best practice, see #48

[robo9k]

License should've been updated inside the repo and outside of it.