Use cargo-deny in CI

[james7132]

cargo-deny provides a plethora of useful checks to ensure dependencies (and transitive dependencies) stay within the bounds.

For example, twilight is licensed under ISC, having a GPL or even MPL dependency or dependency of a dependency would potentially compromise that.

It also can help block duplicate dependencies from being introduced to avoid lengthening compile times, if that's desirable. It can also check if any of the dependencies in the tree have security advisories (though dependabot also does this), and check if any of them are unmaintained.

Enabling this should only require installing it via cargo install cargo-deny and following it's steps for adding a deny.toml at the top of the repo.