Add nestedListsWithoutBlankLine lever; make blocksInterruptParagraphs interrupt consistently in lists

[dereuromark]

Summary

Adds two composable, opt-in list-nesting levers and deprecates the broad one, so paragraph interruption behaves the same at the top level and inside list items.

• New nestedListsWithoutBlankLine lever: a sublist nests inside a list item without a blank line (markdown-style compact lists). Nothing else nests. Available as a constructor flag, setNestedListsWithoutBlankLine(), and DjotConverter::withNestedListsWithoutBlankLine().
• blocksInterruptParagraphs made consistent: it now also interrupts a list item's lead paragraph, so an indented non-list block nests inside the item, using the same lone-marker lookahead as the top level. Unambiguous openers (#, fenced code, :::, ---) and real multi-line blocks nest; an ambiguous single-line marker (> comparison, |) stays literal. A lone > 5 indented under a list item now behaves exactly like it does at the top level.
• significantNewlines redefined: now equivalent to blocksInterruptParagraphs + nestedListsWithoutBlankLine, and deprecated. The broad nestedBlocksInLists is also deprecated (kept working, unchanged eager nesting).

All levers are opt-in; no default behavior changes.

Implementation

• A single gate BlockParser::allowsImmediateNestedBlock() drives both in-list nesting sites; the interrupt branch routes through the existing startsNewBlockSignificant() lookahead so in-list and top-level interruption share one rule.
• Docs updated to match: parser-options guide (new mode section + comparison tables), API reference, enhancements (optional-modes now lead with the granular levers), and the syntax guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (a46d0a6) to head (547aa13).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #207      +/-   ##
============================================
+ Coverage     91.82%   91.87%   +0.04%     
- Complexity     3458     3470      +12     
============================================
  Files           104      104              
  Lines          9799     9829      +30     
============================================
+ Hits           8998     9030      +32     
+ Misses          801      799       -2     

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