Fix soft hyphens not being preserved when round tripping text through the terminal

Also roundtrip all characters in the Cf category.

Characters with the DI (Default Ignorable) property are now
preserved but not rendered and treated as zero-width
as per the unicode standard.
See https://www.unicode.org/faq/unsup_char.html

docs/changelog.rst

@@ -74,6 +74,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
   Applications must use the dedicated escape code to turn on the protocol.
   (:iss:`4075`)
 
+- Fix soft hyphens not being preserved when round tripping text through the
+  terminal
+
 
 0.23.1 [2021-08-17]
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