Add notebook open protocol handler (#6093)

Adds a protocol handler for notebook open, which can be used from browsers
Uses extension-based handler support so all URIs must start with `azuredatastudio://microsoft.notebook`

Adds 2 actions:
- `/new` opens a new empty notebook
- `/open` opens a HTTP/S file as an untitled notebook or text document

Sample URL:
```
azuredatastudio://microsoft.notebook/open?url=https%3A%2F%2Fraw.githubusercontent.com%2Fkevcunnane%2Fmsbuild_ads_demo%2Fmaster%2F0_YoAzdata.ipynb
```

extensions/notebook/src/common/utils.ts

@@ -7,6 +7,7 @@ import * as childProcess from 'child_process';
 import * as fs from 'fs-extra';
 import * as nls from 'vscode-nls';
 import * as vscode from 'vscode';
+import * as azdata from 'azdata';
 
 const localize = nls.loadMessageBundle();
 
@@ -135,3 +136,9 @@ function outputDataChunk(data: string | Buffer, outputChannel: vscode.OutputChan
 			outputChannel.appendLine(header + line);
 		});
 }
+
+export function isEditorTitleFree(title: string): boolean {
+	let hasTextDoc = vscode.workspace.textDocuments.findIndex(doc => doc.isUntitled && doc.fileName === title) > -1;
+	let hasNotebookDoc = azdata.nb.notebookDocuments.findIndex(doc => doc.isUntitled && doc.fileName === title) > -1;
+	return !hasTextDoc && !hasNotebookDoc;
+}