src/vs/workbench/api/common/extHostCustomEditors.ts/CustomDocumentStoreEntry/getNewBackupUri
class CustomDocumentStoreEntry:	getNewBackupUri(): URI {
		if (!this._storagePath) {
			throw new Error('Backup requires a valid storage path');
		}
		const fileName = hashPath(this.document.uri) + (this._backupCounter++);
		return joinPath(this._storagePath, fileName);
	}