Add V2 workspace tree view

[philliphoff]

Adds a basic implementation of the V2 workspace tree view. This is mostly a refactoring of the application (i.e. resources) tree view into common base types that work across both. Despite the number of files changed (mostly due to naming), most of the logic is the same as used by the application tree view, just made more generic.

Notable changes (and open for discussion):

We need an expanded "contribution registration" mechanism to accommodate both application and workspace resource providers. Here's what I propose (by way of an example package.json):

• Contributions are split into application and workspace sections, with each having resource provider (i.e. resources) and brand data provider (i.e. branches) sections. Providers are specified using an object that contains (today) just a type property (but could be expanded in the future, such as to help differentiate kinds, etc.).

{
    "contributes": {
        "x-azResourcesV2": {
            "application": {
                "branches": [
                    {
                        "type": "microsoft.storage/storageaccounts"
                    }
                ]
            },
            "workspace": {
                "resources": [
                    {
                        "type": "ms-azuretools.vscode-azurestorage"
                    }
                ],
                "branches": [
                    {
                        "type": "ms-azuretools.vscode-azurestorage"
                    }
                ]
            }
        }
}

[alexweininger]

For application branches and resources, I think type should be an AzExtResourceType.

For workspace branches and resources, it can be an arbitrary string.

[philliphoff]

I don't disagree necessarily, but I think I'd like to address this as a follow up PR so as to not get mixed up with all the rest, and give me time to think about this.