Support compressing KTX2 textures without CLI

**Is your feature request related to a problem? Please describe.**
I'm in the process of updating from 2.1.7 to 2.4.2 (due to the toktx command changes), but running into what looks like "incorrect but somehow working" use of the library.

In 2.1.7, we were using something like
```
import { NodeIO, PropertyType } from '@gltf-transform/core';
import { draco, toktx } from "@gltf-transform/cli";
...
await document.transform(
// see https://github.com/donmccurdy/glTF-Transform/blob/main/packages/cli/src/transforms/toktx.ts
toktx({ mode: Mode.ETC1S }),
draco(),
);
```

It looks like in 2.4.2 `draco` has moved into `/functions`, but otherwise this still works.

How can we use the simplicity of the toktx command but not depend on the CLI? Might be missing something obvious here!

Start with the toktx transform in packages/cli/src/transforms/toktx.ts, which the issue links, and compare it with the current functions export where draco now lives. Determine what is required to expose equivalent KTX2 compression without depending on the CLI. Done means a programmatic transform can provide the toktx behavior independently of the CLI.

Written by the indexing model from the issue text.