RFR: 8289227: Support for BCP 47 Extension T - Transformed Content

[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.

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Commit messages:
 - IllformedLocaleEx -> LocaleSyntaxEx
 - SystemProperty tests
 - Revived returning Optional
 - Some clean-ups, including making Extension a sealed class.
 - Bring the specialized methods back
 - Using Optional
 - FieldSeparators()/FieldSubtag() -> Fields()
 - 8289227: Support for BCP 47 Extension T
 - T-ext

Changes: https://git.openjdk.org/jdk/pull/9620/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9620&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289227
  Stats: 776 lines in 23 files changed: 600 ins; 135 del; 41 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