src/vs/workbench/api/common/extHostCustomEditors.ts/EditorProviderStore/add
class EditorProviderStore:	private add(type: WebviewEditorType, viewType: string, extension: IExtensionDescription, provider: vscode.CustomTextEditorProvider | vscode.CustomReadonlyEditorProvider): vscode.Disposable {
		if (this._providers.has(viewType)) {
			throw new Error(`Provider for viewType:${viewType} already registered`);
		}
		this._providers.set(viewType, { type, extension, provider } as ProviderEntry);
		return new extHostTypes.Disposable(() => this._providers.delete(viewType));
	}