RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v11]

[Brian Burkhalter]

> Resurrection of the proposal to add a method to obtain the filename extension originated in PR [2319](https://github.com/openjdk/jdk/pull/2319).

Brian Burkhalter 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 14 additional commits since the last revision:

 - 8057113: Add class level apiNote; improve getExtension() specification; remove hasExtension() and replaceExtension()
 - Merge
 - 8057113: Change @since annotation to 20
 - Merge
 - 8057113: Remove superfluous new Object[] statements from test
 - 8057113: Fix getExtension() commcents; improve hasExtension() implementation
 - 8057113: trailing dot yields empty string extension; add test
 - Merge
 - 8057113: String getExtension(String defaultExtension) to Optional<String> getExtension() and other changes
 - 8057113: Clean up trailing whitespace
 - ... and 4 more: https://git.openjdk.org/jdk/compare/e1633ae0...d11ef42a

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/8066/files
  - new: https://git.openjdk.org/jdk/pull/8066/files/4b5166ba..d11ef42a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=10
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=09-10

  Stats: 39579 lines in 1023 files changed: 15300 ins; 12347 del; 11932 mod
  Patch: https://git.openjdk.org/jdk/pull/8066.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/8066/head:pull/8066

PR: https://git.openjdk.org/jdk/pull/8066