[Feature] Edit Address and Subject fields together with email body

[snipsnipsnip]

• Add a toggle.
• Maybe Deno's @std/front-matter fit.
• messengerUtilities API seems useful, but to use it we have to bump requirement from TB128 to TB140.
• Perhaps it would be cleaner to implement [Feature] Allow custom text preprocessing #12 first and provide this as a built-in.