Avoid rescanning malformed author emails

[gaoflow]

Summary:

• anchor the author email regex so malformed email-like values are not repeatedly rescanned from inside the same token
• add a regression test for the reported dotted-domain payload
• add the changelog fragment and contributor entry

Fixes #562.

Tests:

• python -m pytest -q
• python -m mypy
• black --check --target-version py310 feedparser/mixin.py tests/test_author_email.py
• isort --check-only feedparser/mixin.py tests/test_author_email.py
• flake8 feedparser/mixin.py tests/test_author_email.py
• python -m compileall -q feedparser tests
• git diff --check