RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v18]

[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: Remove API note

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Changes:
  - all: https://git.openjdk.org/jdk/pull/8066/files
  - new: https://git.openjdk.org/jdk/pull/8066/files/d8df5141..198ab9a8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=17
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=8066&range=16-17

  Stats: 15 lines in 1 file changed: 0 ins; 7 del; 8 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