Add definition list rendering extension

[soreavis]

Adds support for definition lists (closes #66). cmark-gfm has no native definition lists, so this is a post-process cmark-extra extension (defl.c), in the same style as the existing heads / inlineimage extensions — the cmark engine itself is untouched.

A term on its own line followed by one or more : description lines is rewritten into <dl>/<dt>/<dd>:

Apple
: Pomaceous fruit of plants of the genus *Malus*.

Markdown
: A lightweight markup language.
: Created by John Gruber in 2004.

→

<dl>
<dt>Apple</dt>
<dd>Pomaceous fruit of plants of the genus <em>Malus</em>.</dd>
<dt>Markdown</dt>
<dd>A lightweight markup language.</dd>
<dd>Created by John Gruber in 2004.</dd>
</dl>

Multiple terms in one list and one or more definitions per term are supported, matching the Markdown Guide.

Behaviour

• Off by default. Inline markup in terms and descriptions is preserved (the parsed inlines are moved into the new nodes rather than re-parsed), and it works inside blockquotes.
• A : line only starts a description when it directly follows a term line, so ordinary paragraphs that happen to contain a colon are unaffected.
• The bundled default.css already styles dl/dt/dd, so no CSS change is needed.

How it's exposed

• Preferences UI — a new "Definition list" checkbox in the Extensions grid, wired with bindings exactly like the other extension toggles (value → definitionListExtension, enabled → renderAsCode via NSNegateBoolean).
• CLI — a --definition-list on|off option.
• Settings — a definitionListExtension flag (Codable + app-group defaults), defaulting to off.

Examples

• Adds examples/test-definition-list.md (basic, multiple-descriptions, inline-markup, and blockquote cases).
• Enriches examples/test1.md — the file shown in the Settings live preview — with a "Definition list extension" section (Apple / Markdown / HTML) and its TOC entry, so the effect is visible the moment the checkbox is toggled.

Changed files

• cmark-extra/extensions/defl.{c,h}, extra-extensions.c — the extension and its registration
• QLMarkdown/Settings.swift, Settings+render.swift — the flag and render wiring
• qlmarkdown_cli/qlmarkdown_cli.swift — the CLI option
• QLMarkdown/ViewController.swift, Base.lproj/Main.storyboard — the Preferences checkbox
• examples/test-definition-list.md, examples/test1.md — examples

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Closes #66.