Split out interpreter path related settings from general configuration service

[karrtikr]

Ideally we should separate each setting type to its own component. But for now interpreter path settings has grown to have a lot of special cases in configSettings.ts.

• It is especially manipulated/validated before use.
• It should be handled asynchronously to solve many of these issues: Remove old code for folder support in interpreter path settings #12452
• In many cases we are forced to use workspace service to access other settings instead of configuration service, due to tangled or cyclic dependency issues with interpreter path settings:

  vscode-python/src/client/extensionActivation.ts

  Line 117 in f530a5c

  // directly queries VSCode API.

  
  

  vscode-python/src/client/interpreter/configuration/interpreterSelector/commands/setInterpreter.ts

  Line 165 in 4bc4dc6

  const defaultInterpreterPathValue = config.get<string>('defaultInterpreterPath');

[karrtikr]

Actually, every component should have it's own settings class. Just having an issue open for interpreter setting seems not so useful. We can do this as and when we split the extension up.