Support envFile option for debugger in launch mode

[hananasen]

Proposal for how to add envFile support.

• add function to parse the env file, can be an absolute or relative path.
• merge environment described in envFile with the ones defined in the config(if there are any), config env vars have precedence.

More context: #119

I am no typescript expert though, probably better ways of solving it. But I really need this in my daily work so figured I would give it a go