feat: Add ability for different symbol per level

[thompson-tomo]

Progresses #263

This enables the symbol used in unordered toc to differ on a per level basis.

This further assists with adoption as the tool can now work with whatever list symbol/s is used in the doc

[Copilot]

Pull request overview

Adds support for specifying different unordered list symbols per TOC nesting level via --entry-prefix, addressing #263’s request to align generated TOCs with a project’s existing markdownlint UL style.

Changes:

• Extend TOC generation to select a bullet symbol per header level from a comma-separated --entry-prefix list.
• Add a transform test covering multi-level symbol cycling (-,*,+).
• Document the new per-level symbol configuration in the README.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
lib/transform.js	Splits entryPrefix into per-level symbols and selects a symbol based on nesting level.
test/transform.js	Adds coverage for per-level bullet symbols in generated TOC output.
README.md	Documents --entry-prefix for single and per-level list symbols and links it in the README TOC.

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