feat: Add image support to Parchment Markdown codec

[CesareIsHere]

📝 Description

Implements bidirectional image support for ParchmentMarkdownCodec. Previously, images were silently dropped during Markdown encoding. Now images are preserved in both directions: Markdown ↔ Parchment.

✨ What's New

• Decode: !alt → BlockEmbed.image(url)
• Encode: BlockEmbed.image(url) → ![](url)
• Round-trip: Images maintain data integrity through conversions

🛠️ Implementation

Decoder

• Added _imageRegExp pattern and _handleImage() method
• Processes images before other block elements

Encoder

• Enhanced handleLine() to detect image embeds specifically
• Converts to ![](url) format while skipping other embeds

🧪 Tests Added

• Basic image parsing (with/without alt text)
• Multiple images
• Images with surrounding text
• Encoding and round-trip conversion

📋 Example

// Decode
final doc = parchmentMarkdown.decode('![](https://example.com/img.jpg)');

// Encode
final markdown = parchmentMarkdown.encode(docWithImage);
// Output: "![](https://example.com/img.jpg)\n\n"

✅ Backward Compatible

• All existing functionality unchanged
• Other embeds still skipped during encoding
• All existing tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.78%. Comparing base (5847b01) to head (28f33c4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #521      +/-   ##
==========================================
+ Coverage   88.69%   88.78%   +0.08%     
==========================================
  Files          63       63              
  Lines       10855    10940      +85     
==========================================
+ Hits         9628     9713      +85     
  Misses       1227     1227              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[amantoux]

@CesareIsHere Thank you for this PR. It is still marked as draft, is this intended?