explorer - add "Open Recent" to web serverless empty explorer

[bpasero]

This makes:

• the existing "Empty Explorer" welcome contributions easier to read by documenting the context keys and rules better [1]
• add a "Open Recent" for vscode.dev [2]

[1] maybe this is a chance to review some of the rules and see if they still apply. The reasoning behind some is really not entirely clear to me.

[2] the condition used here is WorkbenchStateContext.isEqualTo('workspace') and OpenFolderWorkspaceSupportContext.toNegated() which is used in other places to identify vscode.dev (we automatically put the user into an empty workspace with tmp scheme to enable opening local folders without window reload, but we never support opening folders or workspaces otherwise). Compare with a similar commit I pushed to have "Open Folder" map to "Add Folder to Workspace": 12f9c5f

I felt that showing "Open Recent" for every explorer might be a bit too much because the button is quite heavy when you stack 2 of them:

Compared to the current welcome screen (which btw wrongly talks about "Adding Folder" to the workspace):

//cc @isidorn