RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v19]

[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 24 additional commits since the last revision:

 - 8057113: Revise specification and align default implementation and test with it
 - Merge
 - 8057113: Remove API note
 - 8057113: Change specification as suggested in review
 - 8057113: Update specification, implementation, and test
 - 8057113: Correct length 2 case; update test cases
 - 8057113: Revert return type back to a nullable-String; update test
 - Merge
 - 8057113: Remove extensions() test method; add threthree new test cases
 - 8057113: Revert excision of wild cards from imports
 - ... and 14 more: https://git.openjdk.org/jdk/compare/9fe1ec51...104a7116

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Changes:
  - all: https://git.openjdk.org/jdk/pull/8066/files
  - new: https://git.openjdk.org/jdk/pull/8066/files/198ab9a8..104a7116

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=18
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=17-18

  Stats: 195188 lines in 3394 files changed: 95414 ins; 79062 del; 20712 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