[Gulliby]

Requested by gulliby

Before. Every table-of-contents entry was written as a bullet, so a document whose house style is a numbered contents list had to be renumbered by hand after each run.

After. --numbered writes the entries as an ordered list, numbered per depth among siblings and indented by four spaces per level. It works with generating, --check, --print and --insert; without the flag the output is unchanged. For example:

$ bun run src/cli.ts --print --numbered --min-depth 2 NOTES.md
1. [Installation](#installation)
2. [Usage](#usage)
    1. [Sub](#sub)
    2. [More](#more)
3. [License](#license)

How. src/toc.ts (generateToc and updateToc take a trailing numbered = false) and src/cli.ts (the flag, the usage text, passing the value on). New tests are in tests/numbered.test.ts; two existing help-text assertions got the new [--numbered] text. README gets a "Numbered list" section and an options row. No new dependencies; bun test, typecheck and lint pass.