feat: support multi-line values in metagen

[benlubas]

Allow things like this:

["core.esupports.metagen"] = {
  config = {
    undojoin_updates = true,
    type = "empty",
    template = {
      { "title" },
      { "description" },
      { "authors", "[\n  benlubas\n  Zlare\n]" },
    }
  },
},

which generates:

@document.meta
title: test1
description: 
authors: [
  benlubas
  Zlare
]
@end

This was requested in the discord by @zlare