Make user's timezone available in user_session

[mihidumh]

Is your feature request related to a problem? Please describe.
It would be nice to be able to know the user's time zone as context in addition to language.

Describe the solution you'd like
cl.user_session.get("timezone") will return the user's time zone similar to cl.user_session.get("languages").

[perutilli]

I would also find this useful, since it's often necessary to insert the current date and time as a variable in the prompt.