addHeading method
Adds an HTML section heading element.
Implementation
void addHeading(String text, {int level = 1}) {
const allowedTags = ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'];
final tag = allowedTags.contains('h$level') ? 'h$level' : allowedTags.first;
addRaw(_wrap(tag: tag, content: text), addEOL: true);
}