Provide all selected URIs for command execution

[msujew]

What it does

Closes #10198

VSCode provides in addition to the URI of the last selected file in the explorer also an array of all selected URIs. This change aligns Theia to this behavior.

How to test

1. Install the test extension (source)
2. Select multiple/one file in the file explorer
3. Use the Show URIs command in the context menu of the file explorer
4. It should show the same output as if installed in vscode, including all selected URIs

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirm that the changes work as expected with the provided test extension 👍

[colin-grant-work]

@msujew, I merged #11290 ahead of this and created a conflict, sorry! Now this should be done in the plugin menu command adapter, rather than the menu contribution handler.

[msujew]

@colin-grant-work Thanks for the hint, I was able to rebase easily 👍

[colin-grant-work]

May I suggest this more exact typing?

    protected getSelectedResources(): [CodeUri | TreeViewSelection | undefined, CodeUri[] | undefined] {
        const selection = this.selectionService.selection;
        const firstMember = TreeWidgetSelection.is(selection) && selection.source instanceof TreeViewWidget && selection[0]
            ? selection.source.toTreeViewSelection(selection[0])
            : (UriSelection.getUri(selection) ?? this.resourceContextKey.get())?.['codeUri'];
        const secondMember = TreeWidgetSelection.is(selection)
            ? UriSelection.getUris(selection).map(uri => uri['codeUri'])
            : undefined;
        return [firstMember, secondMember];
    }