RFR: 8343110: Add getChars(int, int, char[], int) to CharSequence and CharBuffer

[Markus KARG]

This Pull Request proposes an implementation for [JDK-8343110](https://bugs.openjdk.org/browse/JDK-8343110): Adding the new method `public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)` to the `CharSequence` interface, providing a **bulk-read** facility including a default implementation iterating over `charAt(int)`.

In addition, this Pull Request proposes to replace the implementation of `Reader.of(CharSequence).read(char[] cbuf, int off, int len)` to invoke `CharSequence.getChars(next, next + n, cbuf, off)` instead of utilizing pattern matching for switch. Also, this PR proposes to implement `CharBuffer.getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)` as an alias for `CharBuffer.get(srcBegin, dst, dstBegin, srcEnd - srcBegin)`.

To ensure quality...
* ...the method signature and JavaDocs are adapted from `AbstractStringBuilder.getChars(...)`.
* ...this PR relies upon the existing tests for `Reader.of(CharSequence)`, as these provide sufficient coverage of all changes introduced by this PR.

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

Commit messages:
 - Merge master
 - Copyright (C) 2025
 - @Override
 - Draft: CharSequence.getChars

Changes: https://git.openjdk.org/jdk/pull/21730/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21730&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343110
  Stats: 79 lines in 5 files changed: 64 ins; 9 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/21730.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21730/head:pull/21730

PR: https://git.openjdk.org/jdk/pull/21730