Function detectFormat

detectFormat(content: string): FeedFormat
Detect feed format from content string.
Parameters
- content: string
  Feed content as string
Returns FeedFormat
Detected format or 'unknown' if format cannot be determined
Remarks
Analyzes the content to determine if it's RSS, Atom, or JSON Feed. Detection is based on root elements, namespaces, and structure.

Detection priority:
1. JSON Feed (checks for JSON with jsonfeed.org version)
2. RSS (checks for <rss> or <rdf:RDF> root elements)
3. Atom (checks for <feed> root element with Atom namespace)
Example
```
const format = detectFormat(feedContent);
if (format === 'rss') {
  console.log('This is an RSS feed');
}
```
- Defined in src/feed/detect.ts:42