Documentation missing links to cross-package sources

[donmccurdy]

Describe the bug

API documentation for @gltf-transform/extensions is missing source links to implementations coming from the @gltf-transform/core package.

To Reproduce

1. Visit https://gltf-transform.donmccurdy.com/classes/extensions.materialsvariants.html#isRequired

2. Scroll to dispose or isRequired definitions

3. Note they have broken source links like

   Defined in glTF-Transform/packages/core/dist/extension.d.ts:67

Expected behavior

Source links should be correct and clickable.

Additional context

• #342