Drop lone-marker guard: markers interrupt under blocksInterruptParagraphs

[dereuromark]

Drop lone-marker guard under blocksInterruptParagraphs

blocksInterruptParagraphs (and its significantNewlines alias) is an opt-in
markdown/chat-like mode. The lone-marker lookahead added to it suppressed
genuine blocks, not just ambiguous prose: because a quote or list with
lazy continuation is byte-for-byte identical to a wrapped comparison, the
guard silently dropped legitimate content with no blank line before it.

foo
> a real quote
that wraps onto the next line

became one paragraph instead of a blockquote; single-line and lazily
wrapped lists were lost the same way.

A user enabling this mode is asking for aggressive interruption, so a
line-leading marker now interrupts the paragraph again, with no lookahead:

• -/*/+ and > interrupt unconditionally (at top level and inside
  list items, so a sublist nests there too).
• | interrupts only when the line is a real table row
  (tableParser->isTableRow()), so a pipe in prose does not split a
  paragraph into stray blocks.
• # headings, fences and divs are unchanged; ordered lists keep the
  CommonMark 1./1)-only rule.

The cost is that an ambiguous x = 5\n- 3 or if a\n> b is read as a
block in this mode, which is the expected bargain of an opt-in
markdown-like mode. The default (non-blocksInterruptParagraphs) behavior,
where no block interrupts a paragraph, is unchanged.

nestedListsWithoutBlankLine remains the smaller subset: it nests sublists
in list items but does not let other block types interrupt. Tests that
encoded the old lone-marker guard are updated to the new contract.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.88%. Comparing base (efee942) to head (c2a6530).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #210      +/-   ##
============================================
+ Coverage     91.87%   91.88%   +0.01%     
+ Complexity     3488     3470      -18     
============================================
  Files           104      104              
  Lines          9860     9839      -21     
============================================
- Hits           9059     9041      -18     
+ Misses          801      798       -3     

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