refactor: remove CSI unescaping and clean up related names and comments

src/nvim/input.c

@@ -105,7 +105,7 @@ int get_keystroke(MultiQueue *events)
     // terminal code to complete.
     n = os_inchar(buf + len, maxlen, len == 0 ? -1L : 100L, 0, events);
     if (n > 0) {
-      // Replace zero and CSI by a special key code.
+      // Replace zero and K_SPECIAL by a special key code.
       n = fix_input_buffer(buf + len, n);
       len += n;
       waited = 0;