Refactor `content.text` options into separate content providers

[traut]

Design

Refactor format_as options from content.text into separate content providers:

instead of

content text {
   text = "some title"
   format_as = "title"
}

we would have

content title {
  value = "some title"
}

Notes

• use value attribute instead of text used in the content.text block
• type-specific attributes move into a dedicated content providers:
  • absolute_title_size becomes absolute_size and goes into content.title
  • code_language becomes language and goes into content.code