gh-154730: Add timeout in PyREPL to distinguish between bare Esc and multibyte escape sequence

[tanloong]

This adds a timeout to disambiguate bare Esc from the start of a longer escape sequence.

1. Add pending() to BaseEventQueue: returns True when the queue has consumed a prefix (e.g., \x1b) but is still waiting for more bytes.
2. Add flush() to BaseEventQueue: finalises a pending prefix as a standalone key (emits \x1b and re-feeds any remaining buffered bytes).
3. Add esc_timeout parameter to BaseEventQueue (default 0.1 seconds; override via PYREPL_ESC_TIMEOUT env var).

In both unix_console.get_event() and windows_console.get_event(), when event_queue.pending() is True, the next read blocks only for esc_timeout seconds. If the timeout expires, call flush() and deliver the standalone Esc event.

• Issue: PyREPL: ESC key in isearch mode has ambiguous prefix delay #154730