RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v6]

[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: String getExtension(String defaultExtension) to Optional<String> getExtension() and other changes

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8066/files
  - new: https://git.openjdk.java.net/jdk/pull/8066/files/a2573adf..8ed8b1b2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=04-05

  Stats: 31 lines in 3 files changed: 0 ins; 3 del; 28 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8066.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8066/head:pull/8066

PR: https://git.openjdk.java.net/jdk/pull/8066