Create basic SBEnvironment class

Summary: Inspired by https://reviews.llvm.org/D74636, I'm introducing a basic version of Environment in the API. More functionalities can be added as needed.

Reviewers: labath, clayborg

Subscribers: mgorny, lldb-commits, diazhector98

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76111