Add todo and til functions

Add helper functions to write text input into a `TODO.txt` or `TIL.txt`
file respectively.

Extracting this to a function means I'm not able to overwrite the
contents of a file by typing `echo "foo" > TODO.txt` or similar
accidentally and overwriting the entire file instead of appending to it.

lib/shared/modules/zsh.nix

@@ -82,6 +82,24 @@
         fi
       }
 
+      til() {
+        if [[ ''${#} < 1 ]]; then
+          echo "No text was provided."
+          return
+        fi
+
+        echo ''${1} >> TIL.txt
+      }
+
+      todo() {
+        if [[ ''${#} < 1 ]]; then
+          echo "No text was provided."
+          return
+        fi
+
+        echo ''${1} >> TODO.txt
+      }
+
       ttyper() {
         command ${pkgs.ttyper}/bin/ttyper --language english1000 --words 50 "''${@}"
       }