Substitute environment variables in extension settings

[dhruvmanila]

Summary

This PR updates the resolveVariables to consider environment variables.

Reference: https://github.com/microsoft/vscode-black-formatter/blob/458ed8f03f03c6e021361b7dcf4a8c7ab7c3d5fb/src/common/settings.ts#L64-L71

fixes: #448

Test Plan

Note that you'll need to launch the VS Code from the shell so that the VS Code process can see the variables which are present in an activate virtual environment.

$ echo $VIRTUAL_ENV
/Users/dhruv/playground/ruff/.venv

$ code --extensionDevelopmentPath=/Users/dhruv/work/astral/ruff-vscode .

Using the following settings:

{
  "ruff.interpreter": ["${env:VIRTUAL_ENV}/bin/python"]
}

Logs:

2024-07-24 09:15:11.563 [info] Using interpreter: /Users/dhruv/playground/ruff/.venv/bin/python
2024-07-24 09:15:11.634 [info] Using the Ruff binary: /Users/dhruv/playground/ruff/.venv/bin/ruff
2024-07-24 09:15:11.640 [info] Resolved 'ruff.nativeServer: auto' to use the native server
2024-07-24 09:15:11.642 [info] Found Ruff 0.5.4 at /Users/dhruv/playground/ruff/.venv/bin/ruff
2024-07-24 09:15:11.642 [info] Server run command: /Users/dhruv/playground/ruff/.venv/bin/ruff server
2024-07-24 09:15:11.642 [info] Server: Start requested.