Defer writing to writeBundle() or generateBundle()

[donmccurdy]

The plugin API offers these methods:

https://github.com/nytimes/bundler-plugin-gltf/blob/5b252ca48c11ba97233bb37af96ececc3be90a71/packages/core/core.js#L151-L152

We aren't using them now, writing during calls to load(id). Just based on the names, it sounds like maybe we're supposed to be deferring writes to desk until later, but I haven't seen any issues with the current setup. Low-priority, but we should look into the correct use of the plugin API here.