Move format_flowed to a separate crate

This makes it possible to fuzz test the functions
without exposing the module interface in the deltachat core
interface.

Also ensure that format_flowed will not grow a dependency
on deltachat core types.

Cargo.lock

@@ -887,6 +887,7 @@ dependencies = [
  "encoded-words",
  "escaper",
  "fast-socks5",
+ "format-flowed",
  "futures",
  "futures-lite",
  "hex",
@@ -1445,6 +1446,10 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "format-flowed"
+version = "1.0.0"
+
 [[package]]
 name = "futures"
 version = "0.3.25"