Add Creole-style |= table header syntax

[dereuromark]

Summary

Adds support for marking header cells with |= prefix instead of requiring a separator row. This is simpler and more intuitive, especially for users coming from wiki markup (Creole).

Features

• |= Cell marks a header cell
• |=< Left-aligned header
• |=> Right-aligned header
• |=~ Center-aligned header

Example

|= Name |= Age |
| Alice | 28   |
| Bob   | 34   |

Renders as:

<table>
<tr><th>Name</th><th>Age</th></tr>
<tr><td>Alice</td><td>28</td></tr>
<tr><td>Bob</td><td>34</td></tr>
</table>

Backwards Compatible

The traditional separator row syntax continues to work unchanged:

| Name  | Age |
|-------|-----|
| Alice | 28  |

Related

• Discussion: Simpler table markup? jgm/djot#354

Test plan

• Added unit tests for basic |= header syntax
• Added tests for alignment markers (|=<, |=>, |=~)
• Added tests for mixed header/regular cells
• Verified all 812 existing tests still pass
• Manual verification of HTML output

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.33%. Comparing base (94e7a96) to head (09c86ed).

Additional details and impacted files

@@             Coverage Diff              @@
##             master       #8      +/-   ##
============================================
+ Coverage     92.28%   92.33%   +0.05%     
- Complexity     1406     1411       +5     
============================================
  Files            53       53              
  Lines          3616     3640      +24     
============================================
+ Hits           3337     3361      +24     
  Misses          279      279              

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