RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v15]

[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 incrementally with one additional commit since the last revision:

  8057113: Correct length 2 case; update test cases

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Changes:
  - all: https://git.openjdk.org/jdk/pull/8066/files
  - new: https://git.openjdk.org/jdk/pull/8066/files/23ca3f10..3f510ecc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=13-14

  Stats: 35 lines in 2 files changed: 10 ins; 19 del; 6 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