Respect python.defaultInterpreterPath setting

[seeM]

We don't currently respect the python.defaultInterpreterPath setting (see upstream description):

1. When initially selecting an interpreter for a session. This is particularly an issue in workspaceless sessions (see How to set a default kernel for jupyter notebook in Positron? #6244 (comment)).
2. When changes are made at the user-scope (we could listen to the IInterpreterPathService.onDidChange event).

---

The logic for our initial interpreter selection is here:

positron/extensions/positron-python/src/client/positron/discoverer.ts

Lines 42 to 47 in 1edb801

	let recommendedInterpreter = interpreterSelector.getRecommendedSuggestion(suggestions, workspaceUri)
	?.interpreter;
	if (!recommendedInterpreter) {
	// fallback to active interpreter if we don't have a recommended interpreter
	recommendedInterpreter = await interpreterService.getActiveInterpreter(workspaceUri);
	}

We can get the value of the python.defaultInterpreterPath setting either from PythonSettings.pythonPath or IInterpreterPathService.get (which I think is used to set pythonPath).

One challenge is how to determine whether the result of interpreterSelector.getRecommendedSuggestion should be ignored in favor of defaultInterpreterPath. They should be doing something similar upstream so there should be a solution.

[sharon-wang]

Adding this to #6202, as we intend to document python.defaultInterpreterPath for Workbench admins as the method to set a default interpreter for users.