NB: it's PYTHONPATH/pythonpath not PYTHON_PATH
Might be good to call the other setting python_exe or similar to avoid confusion with pythonpath? (python's fault for picking a confusing name :) )

Changed param

Added

		"configuration": {
			"title": "Python settings for Pysys",
			"properties": {
				"pysys.interpreter_path": {
					"type": "string",
					"description": "Path to Python that has Pysys installed. This overrides the default of using python on the PATH."
				},
				"pysys.PYTHONPATH": {
					"type": "string",
					"description": "PYTHONPATH will be *Added* to the environment when running pysys."
				}
			}

interpreter_path will now effect the python used to invoke pysys.

Need to add handling for PYTHONPATH so that it will be added to the environment when the pysys command runs.

We should also consider fitting in with what the Python VSCode plugin does e.g. use a .env file and/or venv (https://code.visualstudio.com/docs/python/environments#:~:text=In VS Code, PYTHONPATH can be set through,is routed through the terminal such as debugging) - would be good to recommend a single approach that would work for both, since they'll almost always be used together