Tree drag and drop API proposal should support dragging into the editor

[alexr00]

The tree drag and drop proposal allows extensions to provide data transfer items when a tree item is dragged, but the only way to drag into the editor is to attach a resource to the tree item.

Attaching a resource to a tree items means that the tree item will receive resource related display characteristics (decorations colors for example). This is not always desirable.

In the existing handleDrag API proposal, we should allow extensions to provide a resourceurls mime type that the editor then knows how to handle when the tree item is dropped there. This resourceurls type is already used internally.

[alexr00]

Closed by #140709

[alexr00]

This can be verified with a small change to the tree-view-sample.

The handleDrag method in the drag and drop test view in that sample adds a "text/treeItem" mime type currently:

https://github.com/microsoft/vscode-extension-samples/blob/799d4ab2499ff4d70361e77a1bfde4388114a79b/tree-view-sample/src/testViewDragAndDrop.ts#L94-L96

You can modify that method to include "resourceurls" by adding "resourceurls" to the supportedMimeTypes in the sample and making the following change:

	public async handleDrag(source: Node[], treeDataTransfer: vscode.TreeDataTransfer): Promise<void> {
		treeDataTransfer.set('text/treeitems', new TestViewObjectTransferItem(source));
		let fileUri = vscode.Uri.file('D:\\repos\\Microsoft\\vscode\\.vscode\\tasks.json'); // Or a path to a file of your choice
		fileUri = fileUri.with({fragment: 'L3,10'});
		treeDataTransfer.set('resourceurls', new TestViewObjectTransferItem(<any>[fileUri.toString()]));
	}

To verify:

1. Make the above changes to the tree view sample and F5.
2. Try dragging a tree item from the "Test View Drag and Drop" into the editor area.
3. Verify that the file you specified in your "resourceurls" is opened in the editor and that line+column you specified in the fragment is correct.

Note: comments are welcome on the API proposal as well, but as this is a verifcation-needed test and not a full TPI, feel free to constrain your testing only to ensuring that the file opens correctly in the editor.

[rchiodo]

Whoops can't test yet. Well not on insiders.

[rchiodo]

/verified with vscode-oss