Fix whitespace around comments between block elements

[ryanchou1994]

Summary

HTML comments between block elements can keep whitespace nodes from recognizing the neighboring blocks. For example, two paragraphs separated by four comments produce six newlines instead of two.

Resolve the neighboring content nodes while collecting children for conversion, skipping comments, doctypes, and whitespace for boundary checks. The traversal advances through siblings once and preserves the input BeautifulSoup tree.

Fixes #193.

Validation

• Regression tests fail on the base revision and pass with this change.
• Full test suite: 98 passed on macOS (Python 3.12.13).
• Flake8, README lint, both mypy checks, and wheel/sdist build passed.
• Linux (Python 3.8.20): tox -e py38 passed, including 98 tests, Flake8 and README lint; both mypy checks and wheel/sdist build passed.
• Installed-wheel API and CLI smoke tests passed for both newline and space-separated comment runs.
• Tests cover comment runs, inline spacing, preformatted content, and repeated conversion without changing the input tree.

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