src/vs/platform/configuration/common/configurationModels.ts/Configuration/keys
class Configuration:	keys(workspace: Workspace | undefined): {
		default: string[];
		user: string[];
		workspace: string[];
		workspaceFolder: string[];
	} {
		const folderConfigurationModel = this.getFolderConfigurationModelForResource(undefined, workspace);
		return {
			default: this._defaultConfiguration.freeze().keys,
			user: this.userConfiguration.freeze().keys,
			workspace: this._workspaceConfiguration.freeze().keys,
			workspaceFolder: folderConfigurationModel ? folderConfigurationModel.freeze().keys : []
		};
	}