Include client's time zone as part of `Page` object

[FeodorFitsner]

Discussed in https://github.com/flet-dev/flet/discussions/4175

More about time zone in Flutter:

• https://stackoverflow.com/questions/13/determine-a-users-timezone
• https://api.dart.dev/stable/3.5.4/dart-core/DateTime/timeZoneOffset.html

We should pass two values from a dart client (which runs in the browser when app is a web app):

• date.timeZoneName - string
• date.timeZoneOffset - Duration

On Python side we create Flet implementation of datetime.tzinfo class which requires offset, DST adjustment (not sure about that) and zone name.

Later on in a user code tzinfo can be used with datetime, e.g. datetime.datetime.now(tz)