Add ability to list sessions and connect to an existing sessions

Adds new methods:

• puppeteer.sessions(): lists current open sessions/browsers. Sessions with a connectionId already have a devtools connection from a worker
• puppeteer.connect(endpoint, sessionId): connects to an open session/browser
• puppeteer.history(): lists recent sessions, open and closed, useful to get a sense of what's happening
• puppeteer.limits(): return current limits (eg. can the user open a new browser now?)