Python: implement `recommendedWorkspaceRuntime()`

[sharon-wang]

Implementation Summary

This is the Python equivalent of #6208.

Implement recommendedWorkspaceRuntime() for the Python extension here:

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

Lines 97 to 104 in 4547a45

	/**
	* Recommend a Python language runtime based on the workspace.
	*/
	async recommendedWorkspaceRuntime(): Promise<positron.LanguageRuntimeMetadata | undefined> {
	// TODO: This is where we could recommend a runtime based on the
	// workspace, e.g. if it contains a virtualenv
	return undefined;
	}

The method returns a single language runtime metadata based on a few criteria:

• has the user indicated a preferred runtime in some way?
• does the workspace contain files that indicate the preferred runtime?
  • .venv or .conda present?
  • other files that indicate the python interpreter to use?
• does the user have a default Python runtime set? (there's a bug re: default python; see Respect python.defaultInterpreterPath setting #6281)

The startup behaviour of the runtime will also be set for the runtime metadata, based on the following:

• if a specific interpreter is indicated in a .venv/.conda/etc, then use StartupBehavior.Immediately
• if the Workspace or User level default interpreter is recommended, then use StartupBehavior.Implicit
• maybe we need to do this? check the global and/or Python-specific startup behaviour to see if StartupBehavior.Explicit may need to be used

[sharon-wang]

Chatted with @isabelizimm -- we determined that there's quite a bit of overlap with #6281 / #6414, so this will be implemented as part of or a fast-follow of #6414.