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.

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Changes
 - Validate signatures in try_decrypt() even if the message isn't encrypted #3859
 - Don't parse the message again after detached signatures validation #3862
+- Move format=flowed support to a separate crate #3869
 
 ### API-Changes