Add multi-line table cell support via + prefix continuation

[dereuromark]

Summary

Alternative implementation to #57 based on jgm's feedback in jgm/djot#368

Syntax

Uses a leading + prefix for continuation lines:

| Name | Description |
|------|-------------|
| Item | Long text   |
+      | continued   |

Why This Approach

jgm (djot creator) suggested:

"use a different delimiter at the beginning to signal that the contents of this line just get added to the cells... this one makes the table look more like a table"

This implementation uses + as the leading delimiter.

Features

• Row-level continuation (+ prefix instead of |)
• Support for multiple continuation lines
• Works with row and cell attributes
• Preserves alignment from separator row
• Content merged with space (like soft breaks in paragraphs)
• Inline formatting (emphasis, links, code spans) works in merged content
• Multi-line captions already supported without special syntax

Edge Cases Tested

• Code spans in continuation lines
• Emphasis spanning continuation
• Links split across continuation (parser handles gracefully)
• Multi-line table captions

Related

• Alternative to Add multi-line table cell support via backslash continuation #57
• Upstream discussion: RFC: Multi-line table cells via backslash continuation jgm/djot#368
• Can be combined with Add table rowspan and colspan support via ^ and < markers #66 (rowspan/colspan via ^ and < markers)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.51%. Comparing base (6c0ff92) to head (0df6a21).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #65      +/-   ##
============================================
+ Coverage     93.48%   93.51%   +0.03%     
- Complexity     1956     1966      +10     
============================================
  Files            74       74              
  Lines          5247     5276      +29     
============================================
+ Hits           4905     4934      +29     
  Misses          342      342              

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[dereuromark]

Closing in favor of #67