feat: add a way to include tag attributes without values on plugins

[stafyniaksacha]

Change HtmlTagDescriptor to allow attribute's values to be boolean

so this

{
  tag: 'link',
  attrs: {
    rel: 'preconnect',
    href: 'https://fonts.gstatic.com/',
    crossorigin: true,
  }
}

note: setting value to false will skip the attribute

html:

<link rel="preconnect" href="https://fonts.gstatic.com/" crossorigin />