v0.1.20 — non-blocking typing + cancel

v0.1.20

• browser_type returns immediately ("typing started"), keyboard dispatch runs in background
• Long text (>= 30 chars) uses single CDP Input.insertText instead of char-by-char
• New browser_cancel_typing({page}) stops an in-flight typing task; already-typed characters remain
• One active typing token per page; starting a new type cancels the previous one
• INSERT_TEXT_THRESHOLD = 30