fuzz: add plist writer API fuzzer

[JeewoongKim]

This PR adds a new fuzz target for libplist's serializer (writer) API.

The existing fuzz targets mainly exercise parser entry points and free the parsed
plist tree. This target complements them by parsing input with
plist_from_memory() and then exercising plist_write_to_string() with an
output format and writer options selected from trailing input bytes.

PLIST_FORMAT_BINARY is intentionally excluded because
plist_write_to_string() does not handle binary output.

Validation:

• Ran a 5-hour AFL++ experiment with no observed crashes or timeouts.
• Compared function-level coverage against all existing parser fuzz targets and
  confirmed additional writer-path coverage.

After this is merged, I will send a follow-up PR to google/oss-fuzz to enable this target
in the libplist OSS-Fuzz build.