Public API follow-up: quote content, fence helpers, answers reader, block lines

[dvejsada]

Summary

#90 made almost everything legaldown-render needs public. After adopting 0.3.0, the renderer still imports these from private modules (its validator_bridge.py):

Import	Used for
legaldown.parser.quote_content, MAX_QUOTE_DEPTH	A block quote's content blocks, as the validator reads them. Block holds a quote only as text.
legaldown.markdown.FENCE_OPEN_RE, closes_fence, dedent, indent_width	A code block's content and info string, read the way the parser reads a fence.
legaldown.cli._read_answers	Reading an answers file (YAML/JSON) for assemble(), with the same errors as legaldown assemble.

One more gap: a block's line. Since #85, the validator's diagnostics name their line. The renderer's own diagnostics (render-ref-ambiguous, ref-not-enumerated, render-not-processed) can't, because the SourceMap/Locator that finds a block's line is private. PlacedMarker.line covers markers only.

Proposal

• Export quote_content and MAX_QUOTE_DEPTH from legaldown. Better still, give a quote block its content blocks in the model, the way Keep nested list structure in the model and when serializing (#16) #61 did for list items.
• Expose a code block's info string and content, either as Block fields or through one public helper, so a consumer needs no fence helpers.
• Export a public read_answers(path).
• Give a public way to find the line of a block or a list item in a parsed document. For example: ValidationResult.line_of(section, block, item=None), or line on Block/ListItem.

The drafting-note content (#88) is related; the rest of U3 is done.

Found while adopting 0.3.0 in legaldown-render.