switch from \1xb to \e in the code

Using `\e` is clearer and shorter than `\x1b`. It's also consistent with how
we write related control chars; e.g., we don't write `\x0a` we write '\n'.

src/reader.cpp

@@ -2719,7 +2719,7 @@ const wchar_t *reader_readline(int nchars) {
                 break;
             }
             // Escape was pressed.
-            case L'\x1b': {
+            case L'\e': {
                 if (data->search_mode) {
                     data->search_mode = NO_SEARCH;