RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v10]

[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 12 additional commits since the last revision:

 - 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
 - 8057113: Implement reviewer suggestions for replaceExtension()
 - 8057113: Add hasExtension() and replaceExtension()
 - ... and 2 more: https://git.openjdk.org/jdk/compare/a64cfe82...4b5166ba

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/8066/files
  - new: https://git.openjdk.org/jdk/pull/8066/files/629d9916..4b5166ba

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=09
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=08-09

  Stats: 345647 lines in 4437 files changed: 228280 ins; 91553 del; 25814 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