(maint) Extract all used environment variables

[AdmiringWorm]

Description Of Changes

These changes extracts all of the environment variables that we
make use of while running Chocolatey CLI. Both variables that are
used as part of running package scripts, and variables we use
only when they are set on the system.

A change to make these environment variables and all other internal
types available to other products and the test projects has been made.

Motivation and Context

To be able to have a single location to add and use environment variable names from.

Testing

N/A. Need to be ran through test kitchen.

Operating Systems Testing

• Windows 10

Change Types Made

• Bug fix (non-breaking change).
• Feature / Enhancement (non-breaking change).
• Breaking change (fix or feature that could cause existing functionality to change).
• Documentation changes.
• PowerShell code changes.

Change Checklist

• Requires a change to the documentation.
• Documentation has been updated.
• Tests to cover my changes, have been added.
• All new and existing tests passed?
• PowerShell code changes: PowerShell v2 compatibility checked?

Related Issue

N/A

[AdmiringWorm]

Putting this back to draft, as we can not make all internal types available for dependent projects.