RFR: 8307547: Support variant collations [v4]

[Alan Bateman]

On Thu, 11 May 2023 20:51:37 GMT, Naoto Sato <naoto at openjdk.org> wrote:

>> The fix to https://bugs.openjdk.org/browse/JDK-8306927 switched the default collation for Swedish to the modern one. In order to provide a means for users who need the old collation, this PR intends to make `Collator` recognize the `co` Unicode locale extension so that multiple implementations for a locale can be provided. I would also like reviews for the corresponding CSR.
>
> Naoto Sato has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Added "reformed" tests

Marked as reviewed by alanb (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/13917#pullrequestreview-1425494382