Add option for the sender address to appear / not appear above the receiver details

[cook-f]

The appearance of Anja Ahlsen... above the receiver details I want to be able to disable and as far as I can tell I can't currently disable it, playing around with the code on my own I can see it's quite simple to do so, so I'm more than happy to make PR for this

[cpg314]

@Sematre What would you think about the following patch, adding a sender-box attribute to the letter-simple method that allows customizing the sender box (above the recipient)? For example, the box can be deactivated passing

  sender-box: sender => none,

diff --git a/src/lib.typ b/src/lib.typ
index b18282e..543e36a 100644
--- a/src/lib.typ
+++ b/src/lib.typ
@@ -501,6 +501,7 @@
     address: none,
     extra: none,
   ),
+  sender-box: sender => sender-box(name: sender.name, sender.address),

   recipient: none,

@@ -566,7 +567,7 @@
     )
   }

-  let sender-box      = sender-box(name: sender.name, sender.address)
+  let sender-box      = sender-box(sender)
   let annotations-box = annotations-box(annotations)
   let recipient-box   = recipient-box(recipient)

One might argue that this is a case for letter-generic over letter-simple, but I think this customization is common enough.