Improve documentation for writing custom extensions

[donmccurdy]

See discussion in https://github.com/donmccurdy/glTF-Transform/issues/543 — documentation for writing custom implementations of extensions, without or without property references, would be a very useful addition to the library. Particularly, how to extend the base Extension and ExtensionProperty classes correctly. Note that there are some subtle differences to doing this in JavaScript vs. TypeScript, which should also be discussed.

Related:

• https://gltf-transform.donmccurdy.com/extensions.html
• https://gltf-transform.donmccurdy.com/classes/core.extension.html
• https://gltf-transform.donmccurdy.com/classes/core.extensionproperty.html