RFR: 8353795: Add Writer.of(StringBuilder)

[Markus KARG]

On Sat, 5 Apr 2025 17:36:29 GMT, Markus KARG <duke at openjdk.org> wrote:

> 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`.

@liach How to proceed?

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PR Comment: https://git.openjdk.org/jdk/pull/24469#issuecomment-2849236880