Honor multiline JSON for BoxList string input and output

[shguild]

When BoxList.to_json(multiline=True) is called without a filename, it currently returns a JSON array instead of one JSON value per line. Likewise, BoxList.from_json(json_string, multiline=True) ignores the flag and raises JSONDecodeError: Extra data for multiple JSON lines. Both operations already work with files.

This change honors multiline=True for string input and output, matching the existing file behavior, including blank/comment lines and universal newlines. The default JSON array behavior stays unchanged.

from box import BoxList

values = BoxList([{"a": 1}, {"b": 2}])
text = values.to_json(multiline=True)
assert text == '{"a": 1}\n{"b": 2}'
assert BoxList.from_json(text, multiline=True) == values

Added 15 regression cases covering empty input, mixed values, Unicode line separators, CR/LF/CRLF, comments, JSON options, invalid JSON, and default array behavior. On the original code, 11 fail and 4 pass.

Validation on Windows / Python 3.12.14:

• python -X utf8 -m pytest --cov=box -q: 174 passed, 92% total coverage. UTF-8 mode is needed for an existing TOML fixture on this Windows installation.
• Black 24.10.0 checks pass for the three changed Python files.
• mypy box: success (8 source files).
• Cython translation passes for both changed modules; native compiled extensions were not built locally.
• git diff --check passes.

Includes changelog and contributor credit as requested by CONTRIBUTING.rst. Targets develop.

Prepared with Codex (GPT-6) assistance.