RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v4]

[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: Implement reviewer suggestions for replaceExtension()

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8066/files
  - new: https://git.openjdk.java.net/jdk/pull/8066/files/4adf810d..04ccbb64

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=02-03

  Stats: 73 lines in 3 files changed: 17 ins; 24 del; 32 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