fix: implement proper structural serialization for Julia executable folds

[Parth-Garg2629]

Overview

This PR addresses the issue where multiline Julia code in executable folds was being concatenated into a single line, leading to ParseError in the Julia kernel (Issue #14 ).

Following the maintainer's feedback, instead of disabling input converters, we have implemented a dedicated recursive serializer that respects the TeXmacs document structure while preserving formatting capabilities.

Technical Details

The fix is implemented in progs/init-julia.scm through the following changes:

• julia-serialize-sub: A new recursive function that traverses the TeXmacs tree to handle different node types appropriately.
• Newline Preservation: The serializer now explicitly identifies document nodes and joins their children (lines) with \n characters. This ensures that multiline code blocks are sent to Julia with their structural integrity intact.
• Math Conversion: The fix preserves the functionality of plugin-math-input by calling it specifically for math tags. This ensures that mathematical symbols (e.g., <math|\alpha>) are still correctly translated for the Julia kernel (e.g., to alpha).
• Registration: The new structural serializer is registered in the plugin-configure block as the primary method for code output.

This approach ensures that the Julia kernel receives well-formatted, multiline code while maintaining all the benefits of TeXmacs' structured document system.

Verification

• Multiline Execution: Verified that multiline logic in executable folds executes line-by-line without ParseError.
• Symbol Conversion: Verified that mathematical symbols within session lines are still correctly translated using the existing converter logic.

[Parth-Garg2629]

@hammerfunctor could u please check if this meets the requierments you told about earlier