RFR: 8353795: Add Writer.of(StringBuilder)

[Markus KARG]

This Pull Requests proposes an implementation for [JDK-8353795](https://bugs.openjdk.org/browse/JDK-8353795): Adding the new method `public static Writer Writer.of(StringBuilder)`, providing a non-synchronized Writer implementation optimized for writing into `StringBuilder`.

A basic test is provided to proof that the new `Writer` behaves as expected. For comparison, the same test is also run against `StringWriter`.

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Commit messages:
 - Removed unused import
 - Limited to Writer.of(StringBuilder)
 - Must not change implementation of StringWriter as getBuffer would not reflect current content of internally used Writer
 - Fixed typo
 - Do not wrap Writer, but return it as-is
 - Forward flush() and close() for Flushable and Closeable
 - Draft: Test for Writer.of(Appendable)
 - Writer.of(Appendable)

Changes: https://git.openjdk.org/jdk/pull/24469/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24469&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353795
  Stats: 379 lines in 4 files changed: 376 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/24469.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24469/head:pull/24469

PR: https://git.openjdk.org/jdk/pull/24469