Support multi-selection of tree items in tree views

[tsmaeder]

What it does

Adds multi-selection support to tree views contributed via VS Code API

Fixes #9074

Contributed on behalf of STMicroelectronics

How to test

The attached extension contributes a tree view that has multi-select enabled. There is a command "Print Selection" which prints out the selected items on the extension side. Make sure the behavior is consistent with VS Code.

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

[tsmaeder]

Here's the extension:

custom-view-samples-0.0.1.zip

And here's the source:

tree-view-sample.zip

[paul-marechal]

The changes work as described when using the provided extension, although I made some modifications to make sure that inline actions also work properly:

I added the following menu entry in the extension's package.json:

        {
          "command": "treeViewDnD.printSelection",
          "when": "view == testViewDragAndDrop && viewItem == aaaaaa",
          "group": "inline"
        }

Along with editing the following code:

// file: out/testViewDragAndDrop.js
// class: TestViewDragAndDrop

    _getTreeItem(key) {
        const treeElement = this._getTreeElement(key);
        // An example of how to use codicons in a MarkdownString in a tree item tooltip.
        const tooltip = new vscode.MarkdownString(`$(zap) Tooltip for ${key}`, true);
        return {
            // required for `viewItem == X` to work:
            contextValue: key,
            label: /**vscode.TreeItemLabel**/ { label: key, highlights: key.length > 1 ? [[key.length - 2, key.length - 1]] : void 0 },
            tooltip,
            collapsibleState: treeElement && Object.keys(treeElement).length ? vscode.TreeItemCollapsibleState.Collapsed : vscode.TreeItemCollapsibleState.None,
            resourceUri: vscode.Uri.parse(`/tmp/${key}`),
        };
    }

[tsmaeder]

@paul-marechal since you changed some code and I assume you tested it, I'll let you merge the changes.

[paul-marechal]

@tsmaeder most of my changes were trivial, though there was one potential bug fixed by b610af8.

I confirm that I manually tested the changes and it still works as expected.