Fix #4089 Linked account cancel (#5347)

VSCode serialization changed rejection handling to only serialize errors, which caused things to break
- Changed to return either account info or a cancel message in the resolve
- Rewrote to use promises. Tracking how to return canceled through 4+ thenables was way trickier than just using a promise
- Updated unit tests to handle new scenario
- Tested integration tests, realized they a) didn't run and b) didn't passed. 
  - Added vscode dev dependency to fix run issue
  - Fixed tests to account for behavior changes in tree state.

extensions/azurecore/src/test/azureResource/providers/database/databaseTreeDataProvider.test.ts

@@ -142,7 +142,7 @@ describe('AzureResourceDatabaseTreeDataProvider.getChildren', function (): void
 			should(child.tenantId).equal(mockTenantId);
 			should(child.treeItem.id).equal(`databaseServer_${database.serverFullName}.database_${database.name}`);
 			should(child.treeItem.label).equal(`${database.name} (${database.serverName})`);
-			should(child.treeItem.collapsibleState).equal(vscode.TreeItemCollapsibleState.None);
+			should(child.treeItem.collapsibleState).equal(vscode.TreeItemCollapsibleState.Collapsed);
 			should(child.treeItem.contextValue).equal(AzureResourceItemType.database);
 		}
 	});

extensions/azurecore/src/test/azureResource/providers/databaseServer/databaseServerTreeDataProvider.test.ts

@@ -142,7 +142,7 @@ describe('AzureResourceDatabaseServerTreeDataProvider.getChildren', function ():
 			should(child.tenantId).equal(mockTenantId);
 			should(child.treeItem.id).equal(`databaseServer_${databaseServer.name}`);
 			should(child.treeItem.label).equal(databaseServer.name);
-			should(child.treeItem.collapsibleState).equal(vscode.TreeItemCollapsibleState.None);
+			should(child.treeItem.collapsibleState).equal(vscode.TreeItemCollapsibleState.Collapsed);
 			should(child.treeItem.contextValue).equal(AzureResourceItemType.databaseServer);
 		}
 	});