Add parsing args from environment

[AdamYoblick]

Fixes #1612

• Added reading args from the environment. Command line args always take precedence.
• Added unit tests for environment args

Testing was done in several ways:

1. I added multiple unit tests for the new code that reads from the environment, as well as made sure that existing tests were passing.
2. I debugged a python file locally, pointing vscode at my local debugpy source, using the following launch.json config:

{
      "name": "Python Debugger: Current File WITH ARGS",
      "type": "debugpy",
      "request": "launch",
      "program": "${file}",
      "console": "integratedTerminal",
      "debugAdapterPath": "E:/repos/github/debugpy/src/debugpy/adapter",
      "args" : ["--msg", "Hello World"]
    },

I verified that these extra args were passed to the file being run and that breakpoints could be hit. I also verified that this worked with no args.

3. I opened a command prompt and starting debugpy in listen mode:

python E:\repos\github\debugpy\src\debugpy --listen 5678 --wait-for-client "C:\Users\advolker\OneDrive - Microsoft\Desktop\pythonTest\infiniteLoop.py"

Then I attached through vscode using the following launch.json config:

    {
      "name": "Attach",
      "type": "python",
      "request": "attach",
      "connect": {
        "port": 5678,
      }  

Breakpoints worked just fine.

4. I did the same test as above, but I set the options --listen 5678 and --wait-for-client in the environment before running debugpy, like this:

set DEBUGPY_EXTRA_ARGV=--listen 5678 --wait-for-client

Then I started debugpy from the same command prompt with only the target specified. The other options were read from the environment:

python E:\repos\github\debugpy\src\debugpy "C:\Users\advolker\OneDrive - Microsoft\Desktop\pythonTest\infiniteLoop.py"

Then I attached through vscode in the same way as the previous step. Breakpoints worked just fine.

[rchiodo]

This seems off? The parameter is being skipped now?

[AdamYoblick]

This was just a refactor with no functional difference. It defaults to false, so in the case of the empty string, the conditional on line 96 will eval to false. But in the case where it has a value, line 96 will eval to true, and the default value will be overridden on line 98.