Allow references in .env file

[Flamefire]

Currently the .env file is not a real dot-env-file because it is a simple list of name=value pairs. However "real" .env supports string references: see https://marketplace.visualstudio.com/items?itemName=mikestead.dotenv

Why is this important?

I set up my PYTHONPATH in the .env file to mirror runtime set env variables so I can use the integrated debugger. This becomes quite lengthy so I tried:

REPO="/home/user/git/foobar"
PYTHONPATH="${REPO}/foo:${REPO}/bar"
# Or
PYTHONPATH="${REPO}/foo"
PYTHONPATH="${PYTHONPATH}:${REPO}/bar"

Would be great, if this was allowed.

[DonJayamanne]

• Use https://www.npmjs.com/package/dotenv-expand to expand variables
• Ensure ${workspaceFolder} gets expanded to current workspace folder
• Ensure we support multi-root workspace folders (all you need to do is use current resource when expanding ${workspaceFolder})

@ericsnowcurrently pointed out that ${xxx} values for env variables will get expanded in the shell (applies to Pwsh, bash, etc). I.e. when using env files in shells, then we don't need to expand them.
However when not using a shell, then we will need this.