Fix numbered list markers in markdown export

[miachillgood]

Summary

Fixes documentToMarkdown so consecutive numbered list nodes export as sequential Markdown markers instead of every item using 1.. The encoder now tracks the displayed numbered-list marker for each contiguous list block and passes it to the numbered-list node parser.

This also preserves an explicit starting marker, such as exporting 3, 4, 5 when the first item starts at 3.

Fixes #1206.

Validation

• Added regression coverage for sequential numbered list export.
• Added regression coverage for an explicit numbered-list start marker.
• I could not run the Flutter/Dart test suite locally because this machine does not have dart or flutter installed.

[CLAassistant]

All committers have signed the CLA.