Add abbreviation definition syntax

[dereuromark]

This PR adds abbreviation definition syntax to djot, enabling automatic wrapping of defined terms in abbreviation elements throughout a document.

Syntax

*[HTML]: Hyper Text Markup Language
*[W3C]: World Wide Web Consortium

When these abbreviations appear in text, they render as:

<abbr title="Hyper Text Markup Language">HTML</abbr>
<abbr title="World Wide Web Consortium">W3C</abbr>

Rationale

1. Accessibility

Screen readers can expand abbreviations for users, improving comprehension. The <abbr> element with a title attribute is the semantic HTML way to provide this.

2. DRY Principle

Currently, the only way to create abbreviations in djot is manually per occurrence:

[HTML]{title="Hyper Text Markup Language"}

For technical documentation with dozens of acronyms appearing multiple times, this is tedious and error-prone. Define-once, apply-everywhere is more practical.

3. Established Syntax

The *[ABBR]: expansion syntax is well-established:

• PHP Markdown Extra (2006) - original implementation
• Pandoc - supports via abbreviations extension
• Python-Markdown - abbr extension
• Maruku, kramdown, Marked - all support this syntax
• djot-php - already implements this

This isn't inventing new syntax—it's adopting a proven pattern.

4. Fits Djot's Design

The syntax mirrors existing djot patterns:

Feature	Syntax
Reference link	[label]: url
Footnote	[^label]: content
Abbreviation	*[ABBR]: expansion

The * prefix distinguishes it from links, and the structure is consistent with other definition-style blocks.

5. Non-Breaking Addition

• New syntax that doesn't conflict with existing constructs
• *[ at line start isn't valid in current djot
• Documents without abbreviation definitions are unaffected

Implementation Notes

• Definitions can appear anywhere in the document
• Matching is case-sensitive and word-boundary aware
• Multiple definitions for different cases (HTML vs html) are valid
• The definitions themselves don't render in output

Prior Art / Requests

• Pandoc #167 - Request from 2012, still referenced
• Pandoc #9227 - Recent discussion on abbreviation syntax
• PHP Markdown Extra documentation: https://michelf.ca/projects/php-markdown/extra/#abbr

Reference Implementation

djot-php implements this syntax and can serve as a reference:
https://php-collective.github.io/djot-php/guide/syntax.html#abbreviations-extension
A corresponding PR for djot.js can follow if this spec change is accepted.