Deprecate and migrate old environment variable settings

[bwateratmsft]

Closes #3539.

Settings migration could be a sticky issue. I want to verify the following, to be confident this feature will not break anything:

• No notification if settings aren't used (covers vast majority of users)
• Setting changes get picked up by context manager
• Verify user -> user settings
• Verify workspace -> workspace settings
• Verify workspace folder -> workspace folder settings
• Verify multiple setting targets combined
• Verify multiple settings combined
• Verify an empty workspace folder setting with non-empty user setting works? Nah