Replace \e with \x1B, as the former is a gcc extension

While supported by gcc and clang, \e is a gcc-specific extension and not
formally defined in the C or C++ standards.

See [0] for a list of valid escapes.

[0]: https://stackoverflow.com/a/10220539/17027

src/expand.cpp

@@ -132,7 +132,7 @@ static int is_quotable(const wchar_t *str) {
         case L'\t':
         case L'\r':
         case L'\b':
-        case L'\e': {
+        case L'\x1B': {
             return 0;
         }
         default: { return is_quotable(str + 1); }