Create terminal with environment variables using the new VSC api

[DonJayamanne]

Full control over createTerminal environment#
The new setting TerminalOptions.strictEnv when set to true will not make any modifications to the terminal environment at all, instead it will be used exactly as passed in by the extension.

// This will create a terminal whose environment only contains the single value
// FOO=BAR.
const terminal = createTerminal({
  name: "Test terminal",
  env: {
    "FOO": "BAR"
  },
  strictEnv: true
});

[DonJayamanne]

@qubitron /cc

[DonJayamanne]

When users use the command Python: Create Terminal, we'll use the environment variables from .env files and provide them here.
Based on feedback, we might consider pushing changes upstream to VSC.