[Blazor] Modernize our Blazor JS assets and their delivery mechanism.

* Generate ES6 modules by default. (**P1**)
* This is a pre-condition so that we can reasonably breakdown our bundles into smaller items.
* It enables other toolchains to post-process Blazor in order to perform further transformations.
* It enables us to stop polluting the global window object as libraries can instead use `import` to get a hold to the Blazor instance.
* We will still support opting in to register `window.Blazor` and `window.DotNet` on the document via a startup option.
* This will be one more step towards support for running multiple Blazor instances on the same document.
* Break down Blazor web into multiple assets. (**P1**)
* No concrete numbers for the enhanced SSR features of Blazor web, but it is likely less than 20KB
* We should only pay for the wasm/server JS the moment we use it.