RFR: 8305486: Add split() variants that keep the delimiters to String and j.u.r.Pattern [v5]

[Raffaello Giulietti]

> Add `split()` overloads to `String` and `java.util.regex.Pattern` that, in addition to the substrings returned by current `split()` variants, also return the delimiters matching the regular expression.

Raffaello Giulietti has updated the pull request incrementally with one additional commit since the last revision:

  Small optimization in private split().
  Added tests to compare String and Pattern implementations, in particular for one char regex.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13305/files
  - new: https://git.openjdk.org/jdk/pull/13305/files/178cb645..e7f0980b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13305&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13305&range=03-04

  Stats: 10 lines in 2 files changed: 7 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13305.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13305/head:pull/13305

PR: https://git.openjdk.org/jdk/pull/13305