RFR: 8364007: Add no-argument codePointCount method to CharSequence and String [v15]

[Tatsunori Uchino]

> Adds `codePointCount()` overloads to `String`, `Character`, `(Abstract)StringBuilder`, and `StringBuffer` to make it possible to conveniently retrieve the length of a string as code points without extra boundary checks.
> 
> 
> if (superTremendouslyLongExpressionYieldingAString().codePointCount() > limit) {
>     throw new Exception("exceeding length");
> }
> 
> 
> Is a CSR required to this change?

Tatsunori Uchino has updated the pull request incrementally with one additional commit since the last revision:

  Use `codePointCountSB` and add its test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26461/files
  - new: https://git.openjdk.org/jdk/pull/26461/files/198b3188..585ce36e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26461&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26461&range=13-14

  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26461.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26461/head:pull/26461

PR: https://git.openjdk.org/jdk/pull/26461