Update entrypoints to nest within their platforms

[thekevinscott]

This is another step in the deprecation of the scaffolding step.

Refactor the upscalerjs build process so that each platform (browser, node, and node-gpu) for each flavor (esm, umd for browser, cjs for the nodes) has its own dedicated entry point.

From there we'll be able to further refactor out the platform-specific dependencies to be static imports.