RFR: 8307547: Support variant collations [v4]

[Naoto Sato]

> 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

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13917/files
  - new: https://git.openjdk.org/jdk/pull/13917/files/29b1c8c4..4ab39664

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13917&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13917&range=02-03

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13917.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13917/head:pull/13917

PR: https://git.openjdk.org/jdk/pull/13917