Add Creole-style |= table header syntax

[dereuromark]

Summary

This PR adds Creole-style |= table header syntax to djot-php, providing a more flexible way to mark individual cells as headers.

Features

1. Creole-style Header Cells (|=)

Mark individual cells as headers using the |= prefix (must be directly attached to pipe):

|= 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>

2. Cell Alignment Markers

Alignment markers must be directly attached to |= (no space):

Syntax	Alignment
|=< text	Left
|=> text	Right
|=~ text	Center

Example:

|=< Left |=~ Center |=> Right |
| A      | B        | C       |

Header alignment propagates to data cells below when no separator row is present.

3. Row Headers

This syntax enables row headers on the left side of tables:

|= Category | Value |
|= Apples   | 10    |
|= Oranges  | 20    |

4. Mixed Header/Data Cells

Mix header and regular cells in the same row:

|= Header | Regular |
| Data    | Data    |

Compatibility Notes

Important: The = must be directly attached to the pipe character:

Input	Interpretation
|= text	Header cell with content "text"
| = text	Regular cell with content "= text"
|=<	Left-aligned header
|= <	Header with content "< ..."

Additional compatibility:

• Traditional separator row syntax (|---|) continues to work
• Separator row alignment takes precedence over |= alignment
• Works seamlessly with colspan (<), rowspan (^), and multi-line cells (+)

Test Coverage

• 13 new tests covering all features
• All 1415 existing tests continue to pass
• PHPStan passes with no errors

Related

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.87%. Comparing base (0b8e068) to head (65f247d).

Additional details and impacted files

@@             Coverage Diff              @@
##             master       #8      +/-   ##
============================================
+ Coverage     93.81%   93.87%   +0.06%     
- Complexity     2049     2067      +18     
============================================
  Files            74       74              
  Lines          5495     5552      +57     
============================================
+ Hits           5155     5212      +57     
  Misses          340      340              

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