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.

format-flowed/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "format-flowed"
+version = "1.0.0"
+description = "format=flowed support"
+edition = "2021"
+license = "MPL-2.0"
+
+keywords = ["email"]
+categories = ["email"]
+
+[dependencies]