RFR: 8057113: (fs) Path should have a method to obtain the filename extension [v7]

[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 eight additional commits since the last revision:

 - 8057113: trailing dot yields empty string extension; add test
 - Merge
 - 8057113: String getExtension(String defaultExtension) to Optional<String> getExtension() and other changes
 - 8057113: Clean up trailing whitespace
 - 8057113: Implement reviewer suggestions for replaceExtension()
 - 8057113: Add hasExtension() and replaceExtension()
 - 8057113: Fix variable name in @return verbiage
 - 8057113: (fs) Path should have a method to obtain the filename extension

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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/8ed8b1b2..e5240f1c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8066&range=05-06

  Stats: 206499 lines in 3275 files changed: 147275 ins; 14843 del; 44381 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