Add automatic table of contents generation for Obsidian linter

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What this task asks for

Implement an opt-in rule that generates and updates a TOC from document headings. Implement a new rule, export default `AutoToc` from `src/rules/auto-toc.ts`, that generates or updates a TOC. Opt-in via `<!-- toc -->`. If absent, return input unchanged. The TOC region uses `<!-- toc -->` and `<!-- /toc -->` (case-insensitive, whitespace-tolerant). Ensure blank lines after the start marker, after an optional `title` line, before the end marker, and after the end marker. Include only ATX headings (`#`), filtered by `minLevel`/`maxLevel`.