feat: improve Apple News Format spec completeness

[toc-assistant]

Summary

Improves Apple News Format specification compliance based on cross-referencing with official Apple documentation.

Changes

New Components

• ArticleThumbnail - Custom thumbnail for article feeds (role: article_thumbnail)
• ArticleTitle - Enhanced title component for feeds (role: article_title)

TextComponent Enhancement

• Added additions property for applying links to text ranges
• This enables linking portions of body text without using HTML/Markdown format

MultipartBuilder

• Added WebP support to MIME type detection (image/webp)

Logo Component

• Added explicitContent property (was already present on Photo, Image, Figure, Portrait)

Usage Examples

Text with link additions:

$body = new Body('Check out our website for more info.');
$body->setAdditions([
    [
        'type' => 'link',
        'rangeStart' => 14,
        'rangeLength' => 7,
        'URL' => 'https://example.com',
    ],
]);

Article thumbnail:

$thumbnail = ArticleThumbnail::fromBundle('feed-image.jpg')
    ->setAccessibilityCaption('Article preview image');
$article->addComponent($thumbnail);

Tests

• Added TextComponentAdditionsTest for additions property

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Based on code review against Apple's official documentation at developer.apple.com