`Describe` incorrectly handles single literal types.

[yeoffrey]

Description

It seems that using a single literal in an enums struct will not be interpreted by Describe properly when working with TS types that are literals. Is this the wrong usage of enums or describe? If not, I'm willing to submit a PR for this.

Reproduce

Here is a type which is a union of string literals (working).

import {Describe, enums} from 'superstruct';

type MyType = 'foo' | 'bar';

const MyTypeStruct: Describe<MyType> = enums(['foo', 'bar']);

However, when using Describe with a type which is a single string literal, there is an error.

import {Describe, enums} from 'superstruct';

type MyType = 'foo';

const MyTypeStruct: Describe<MyType> = enums(['foo']);

Error:

Type 'Struct<"foo", { foo: "foo"; }>' is not assignable to type 'Describe<"foo">'.
     Type '{ foo: "foo"; }' is not assignable to type '"foo"'. [2322]

Using Infer

This seems to work the other way when using Infer from a struct to generate a type. For example:

import {Infer, enums} from 'superstruct';

const literalEnum = enums(['foo']);

type MyType = Infer<typeof literalEnum>;

const MyLiteral: MyType = 'foo';

Extra info:

Node: v20.14.0 SuperStruct: v1.0.4 I have strict enabled in my TypeScript configuration for these examples. I'm willing to submit a PR for this.