Fix fish shell integration when prompt function doesn't return a string

Instead of assuming printf will print the prompt, just evaluate the original fish prompt function (which is what fish itself does)

shell/shellIntegration.fish

@@ -20,4 +20,4 @@ if [ "$ISTERM_TESTING" = "1" ]
 	function is_user_prompt; printf '> '; end
 end
 
-function fish_prompt; printf (__is_prompt_start); printf (is_user_prompt); printf (__is_prompt_end); end
+function fish_prompt; printf (__is_prompt_start); is_user_prompt; printf (__is_prompt_end); end