Add wrapped YAML frontmatter support for non-Markdown files

[DandyLyons]

Summary

• add LF-only comment-wrapped YAML frontmatter parsing with built-in and custom syntax presets
• add opt-in non-Markdown support to the six read-only frontmatter commands
• add config schema 0.2.1, publication assets, schema changelog, and user documentation
• diagnose duplicate blocks by file and line while parsing only the first block

Verification

• swift test (1,087 tests)
• python3 scripts/validate-schema-publication.py
• treedocs check --non-interactive
• git diff --check

Closes #98