frontend: add a line editor for shell input

frontend/line-ed/cursor.c

@@ -0,0 +1,27 @@
+#include "line-ed.h"
+#include "cursor.h"
+#include "prompt.h"
+
+void line_ed_coords_to_physical_coords(struct line_ed *ed,
+		unsigned int x, unsigned int y,
+		unsigned int *out_x, unsigned int *out_y)
+{
+	unsigned int prompt_len = 0;
+	if (ed->l_cursor_y == 0) {
+		prompt_len = prompt_length(ed, PROMPT_MAIN);
+	} else if (ed->l_cursor_y <= ed->l_continuations) {
+		prompt_len = prompt_length(ed, PROMPT_CONT);
+	}
+
+	if (y == 0) {
+		x += prompt_len;
+	}
+
+	if (out_x) {
+		*out_x = x;
+	}
+
+	if (out_y) {
+		*out_y = y;
+	}
+}