Add settings for PYTHONPATH and python executable

[CaribouJohn]

To help the user we should allow them to override the default python on the path with their own choice.
This should be explicitly overridden not set by default.

Setting name	Description
pysys-test.python	Path to python executable to use
pysys-test.pythonpath	PYTHON_PATH that will be appended to the environment variable currently set

These settings will replace the default command used when invoking commands. Where we currently use

python -m pysys

If the python executable is set to be

/mypath/bin/python or c:\myWindowsPath\bin\python.exe we should use these

/mypath/bin/python -m pysys ...

[ben-spiller]

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 :) )

[CaribouJohn]

Changed param

[CaribouJohn]

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.

[ben-spiller]

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%20VS%20Code%2C%20PYTHONPATH%20can%20be%20set%20through,is%20routed%20through%20the%20terminal%20such%20as%20debugging) - would be good to recommend a single approach that would work for both, since they'll almost always be used together