The `icu/experimental` feature should probably be `icu/unstable`

[robertbastian]

Unstable is the common name for Cargo features that expose semver unstable code. cargo semver-checks actually has special handling for it, and there doesn't seem to be a way to tell it that the API behind the experimental features is unstable.

~We might also want to get rid of the experimental module and expose the experimental components at the top level. This way, stabilisation will be smoother because paths won't change.~

[sffc]

Related: https://github.com/unicode-org/icu4x/issues/4551, https://github.com/unicode-org/icu4x/issues/6318, https://github.com/unicode-org/icu4x/issues/4193