public final void useFileWriter(String filename, Consumer<T> writerConsumer) {
        useFileWriter(filename, "", writerConsumer);
    }
Gets a previously created {@code CodegenWriter} or creates a new one
if needed.

<p>If a writer already exists, a newline is automatically appended to
the writer (either a newline or whatever value was set on
{@link #setAutomaticSeparator}).

@param filename Name of the file to create.
@param writerConsumer Consumer that is expected to write to the {@code CodegenWriter}.
smithy-codegen-core/src/main/java/software/amazon/smithy/codegen/core/writer/CodegenWriterDelegator.java