Implement envFile (Issue #523)

[chase]

Implemented envFile to close #523

Uses some code from https://github.com/microsoft/vscode-js-debug to parse env files

@testforstephen

[ghost]

All CLA requirements met.

[ShimonBenYair]

@chase

How can I check your PR to see if it works for me ?
Can you please write the steps here

[chase]

@ShimonBenYair Detailed in the test plan here:
https://github.com/microsoft/vscode-java-debug/blob/d3f3b6a068c0a47e21bddb8e3604b24f05166c6f/TestPlan.md#environment-variables

[testforstephen]

overall looks good.

[testforstephen]

LGTM. thanks for your contribution.

[troyspencer]

Thanks for working on this, I've been waiting on somebody to handle that port for a while. I built and installed from this branch and it works great, except the edge case of clicking restart in the debugger uses the values from the previous run instead of loading from the file like pressing start does. Until I realized, my iterations on environment file updates while migrating a dev setup seemed fruitless.