RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v14]

[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 18 additional commits since the last revision:

 - 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
 - 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
 - ... and 8 more: https://git.openjdk.org/jdk/compare/53dfdb9d...23ca3f10

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Changes:
  - all: https://git.openjdk.org/jdk/pull/8066/files
  - new: https://git.openjdk.org/jdk/pull/8066/files/ec417a89..23ca3f10

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=13
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=12-13

  Stats: 135173 lines in 2720 files changed: 82774 ins; 33359 del; 19040 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