setext: Shared safe-Setext policy for CommonMark-family readers

Shared safe-Setext policy for CommonMark-family readers.

myform.setext.setext_headings_enabled(state: StateBlock) → bool

Read the policy from universal, plugin, or source-format options.

myform.setext.parse_safe_setext_heading(state: StateBlock, l0: int, le: int, silent: bool) → bool

Preserve an ambiguous unary nested dash as an empty list item.