Stabilize `feature(int_error_matching)`

[eopb]

I am not very familiar with the stabilization process but I don't know of anything holding back feature(int_error_matching).

The pull request can be found here

[Ravenslofty]

Since it got merged a fortnight ago, I think it's best to wait for it to get used and tested in practice instead of stabilising it right now. It's possible the devs find a better approach for this problem.

[jonas-schievink]

See https://rust-lang.github.io/rustc-guide/stabilization_guide.html for details on the stabilization process.

[jonas-schievink]

Closing in favor of the tracking issue #22639. In order to stabilize this, a libs team member has to start the FCP period.