Adds group feature environment manager UI

[karthiknadig]

Closes #95

[jjjermiah]

@karthiknadig would it also be possible to add a contextValue for the group nodes? I think it would be valuable to execute commands on a group, i.e opening a manifest toml pertaining to that set of environments

[karthiknadig]

@jjjermiah The context for the tree item should include the Group name and manager ID, like this pythonEnvGroup;ms-python.python:venv:mygroup;. You should be able to use that to set commands for specific groups.

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• src/api.ts: Evaluated as low risk

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src/features/views/envManagersView.ts:108

• [nitpick] The variable name 'groupObjects' is ambiguous. It should be renamed to 'environmentGroups' for clarity.

const groupObjects: (string | EnvironmentGroupInfo)[] = [];

src/features/views/envManagersView.ts:128

• Ensure that the new grouping behavior introduced by the 'PythonGroupEnvTreeItem' class is covered by tests.

if (element.kind === EnvTreeItemKind.environmentGroup) {