<i18n dev> RFR: 8289227: Support for BCP 47 Extension T - Transformed Content [v3]

[Naoto Sato]

> This PR is to propose supporting the `T` extension to the BCP 47 to which `java.util.Locale` class conforms. There are two extensions to the BCP 47, one is `Unicode Locale Extension` which has been supported since JDK7, the other is this `Transformed Content` extension. A CSR has also been drafted.

Naoto Sato has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:

 - Modified javadoc of the transformed conent
 - Merge branch 'master' into JDK-8289227-T-ext
 - Removed unnecessary `contains()` check
 - IllformedLocaleEx -> LocaleSyntaxEx
 - SystemProperty tests
 - Revived returning Optional
 - Some clean-ups, including making Extension a sealed class.
 - Bring the specialized methods back
   Some documentation fixes
 - Using Optional
 - FieldSeparators()/FieldSubtag() -> Fields()
 - ... and 2 more: https://git.openjdk.org/jdk/compare/69ea6e10...780f712e

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9620/files
  - new: https://git.openjdk.org/jdk/pull/9620/files/a869efaa..780f712e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9620&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9620&range=01-02

  Stats: 6329 lines in 181 files changed: 3452 ins; 2172 del; 705 mod
  Patch: https://git.openjdk.org/jdk/pull/9620.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9620/head:pull/9620

PR: https://git.openjdk.org/jdk/pull/9620