src/vs/workbench/api/common/extHostCommands.ts/ApiCommandArgument/optional
class ApiCommandArgument:	optional(): ApiCommandArgument<V | undefined | null, O | undefined | null> {
		return new ApiCommandArgument(
			this.name, `(optional) ${this.description}`,
			value => value === undefined || value === null || this.validate(value),
			value => value === undefined ? undefined : value === null ? null : this.convert(value)
		);
	}