Preserve recent multiple-use metadata fields in JSON

[kokotatan]

metadata(name).json currently returns only the first License-File, Import-Name, or Import-Namespace value as a string. For example, metadata containing both License-File: LICENSE and License-File: NOTICE becomes {"license_file": "LICENSE"}, losing the second file.

Add these three fields to the existing multiple-use field set. Core Metadata defines License-File as multiple-use since 2.4, and Import-Name and Import-Namespace since 2.5. PEP 566's JSON conversion requires multiple-use fields to be lists, including when only one value appears.

The public API regression covers single and repeated values, differently cased duplicate headers, preservation of the ; private suffix, and an empty Import-Name. All seven subcases fail before the fix. A Towncrier bugfix fragment is included.

Validation on Windows with Python 3.12:

• Focused JSON tests: 4 passed, 11 subtests passed.
• tox -e py312: 216 passed, 3 skipped, and one pre-existing environment failure in the tests._path.build doctest because this Windows account cannot create symlinks (WinError 1314). The same doctest fails on clean upstream c5acd62.
• The tox run passes mypy (30 files), Ruff, formatting, and package-description checks. _adapters.py has 100% coverage; overall coverage is 98%.
• tox -e docs: Sphinx's warning-as-error build and sphinx-lint pass.

AI assistance: implemented and tested with OpenAI Codex; independently reviewed by a second Codex agent.