feat(agenda): implement support for org_agenda_prefix_format

[swapnilsm]

Summary

This PR implements support for org_agenda_prefix_format, allowing users to customize the prefix of agenda lines using format strings or Lua expressions. The implementation uses a pre-compiled formatting system to ensure high performance and efficiency during agenda rendering.

Related Issues

Closes #913

Changes

• Implementation of org_agenda_prefix_format: Added support for customizing agenda line prefixes, mimicking the Emacs Orgmode feature. Supported in agenda, todo, tags, tags_todo, and search views.
• Pre-compilation & Caching: Format strings are parsed into segments and cached. Lua expressions are compiled once, significantly improving agenda rendering performance compared to a naive per-line parsing approach.
• Support for Lua Expressions: Users can use %(...) placeholders to evaluate custom Lua code within the prefix, with access to headline, item, and metadata.
• Architectural Design: The Formatter module separates parsing from evaluation, leading to cleaner and more maintainable code.
• Semantic Naming: Introduced the marker field in AgendaItem to represent planning status indicators (Deadline, Scheduled, etc.), replacing more ambiguous naming.
• Documentation: Added comprehensive documentation in configuration.org and synchronized the Vim help file.
• Verification: Included a dedicated test suite for the formatter to ensure correctness across various format string configurations.

Checklist

I confirm that I have:

• Followed the Conventional Commits specification
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with make test.
• Checked for breaking changes and documented them, if any.