fix(TS): ProcessData#readCallback type/jsdocs

[ShaMan123]

Description

Fix readCallback type and add docs

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following:

• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Follow the Conventional Commits v1.0.0 standard for PR naming (e.g. feat(examples): add hello-world example).
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[ShaMan123]

Ideally I would do something like:

export type ProcessData = (
  | {
      bytes: Uint8Array;
      readFromCallback?: false;
      readCallback?: never;
    }
  | {
      bytes?: never;
      readFromCallback: true;
      readCallback: ModelLoadCallback;
    }
) & {
  id?: string;
  raw?: boolean;
  progressCallback?: (progress: number, data: ProgressData) => void;
};

But this requires a bit more refactoring since it is referenced as an interface across the codebase.
If you are up for it I can do it.