frontend: line-ed: move platform-specific code to separate directory

frontend/line-ed/line-ed.h

@@ -10,6 +10,7 @@
 #include <ivy/line-source.h>
 #include <stddef.h>
 
+struct s_tty;
 struct s_tty_vmode;
 
 struct line_ed;
@@ -56,6 +57,9 @@ struct line_ed {
 
 	struct ivy_line_source l_line_source;
 
+	/* pointer to tty interface */
+	struct s_tty *l_tty;
+
 	/* the lexical scope that we are currently in.
 	 * this is provided by components further up the input pipeline,
 	 * for example, when we are inside a string or if-statement. */