Add String & Character ASCII case conversion methods

[some-java-user-99206970363698485155 at vodafonemail.de]

Thanks for your effort on deprecating the error-prone toLowerCase() and toUpperCase() methods!

Though that only addresses one aspect of my original proposal. Do you or anyone else have any opinion or feedback regarding the proposed ASCII case conversion methods? Even if you think these methods are not needed, such feedback would already be useful (assuming you explain why you think that is the case, and why the issues I presented are not a problem).

Maybe I will also submit this as "Request for Enhancement" to the bug tracker.

Kind regards

> Hi,
> We discussed this issue on this mailing list[1] earlier this year.
> 
> I investigated the usage of these two methods and found that all use cases within
> JDK are suspicious, resulting in many imperceptible bugs.
> 
> I hope to create a PR for this issue, deprecate these two methods, and create
> alternative methods for them. But I don't have the experience of making such
> changes, maybe I need some guidance or have more experienced people do these things.
> 
> Glavo
> [1] https://mail.openjdk.org/pipermail/core-libs-dev/2023-January/099375.html