Remove the window.AppBoot function

[jsnajdr]

I'm often running into the dreadful "window.AppBoot is not a function error" so I started looking into it.

This PR is the result: let's remove the window.AppBoot function and replace it with a main/boot function that is called directly in the JS code. That's how webpack apps are supposed to be done. Webpack will make sure that the chunks are loaded in the last one, and that the root module which calls main() is run only when all other modules are loaded.

I'm not sure about two things:

1. In addition to loading the entrypoint, we also preload chunks for the current section (e.g., home or reader). These chunks are loaded after the entrypoint. But the scripts will be executed only after main() was already called, because that call is in the entrypoint chunk. Before this PR, we postponed the main call until all chunks were loaded.
2. We might need to revisit @arthur791004's Assets: Fix the order of the chunk files #95748 PR that fixed the order of chunks. The code after this PR is more sensitive about the order.

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-130160

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-130160&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-130160&env=a8c-for-agencies

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~44 bytes removed 📉 [gzipped])

name           parsed_size           gzip_size
entry-login          -43 B  (-0.0%)      -24 B  (-0.0%)
entry-stepper        -32 B  (-0.0%)      -15 B  (-0.0%)
entry-main           -21 B  (-0.0%)       -7 B  (-0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[sirreal]

The reasoning shared in the description makes sense and so do the changes. I did some smoke testing via calypso.live and everything seems to work as expected.

[tyxla]

@jsnajdr did you find a reliable way to trigger the window.AppBoot error locally so we can test whether this solves the issue?