Add ability to specify timezone database location via environment variable

[samuel-emrys]

This adds the ability for a user to populate the TZDATA environment variable to specify the location in which to look for the timezone database, which allows for a runtime query of this location. Without this, date currently only supports compile time determination of the database location.

This facilitates the ability to:

• Use a timezone database provided by a package manager
• Keep the timezone database up to date without re-compilation
• Be flexible about where the timezone database is stored

This is a simplification of https://github.com/HowardHinnant/date/pull/611, removing windows support for a zic-compiled binary database, so that it only adds environmental variable features.

Closes #788