RFR: 8370250: Locale should mention the behavior for duplicate subtags

[Justin Lu]

Please review this PR which is a first stab at clarifying the behavior of duplicate variants, extension singletons, and U extension keys and attributes for BCP47 subtags in the Locale specification. This is a follow up to https://bugs.openjdk.org/browse/JDK-8369739.

Changes are made under the BCP47 and U extension sections that define "well-formed" in the class description.
Additionally, changes are made under the relevant Locale and Locale.Builder methods themselves.

Will update the CSR accordingly when the proposed wording changes are finalized.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/27909/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27909&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370250
  Stats: 31 lines in 1 file changed: 19 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/27909.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27909/head:pull/27909

PR: https://git.openjdk.org/jdk/pull/27909