Use @main attributes

[noppefoxwolf]

It can be used the @main attribute if the filename is App.swift. like SwiftUI.
Is there a reason that filename is main.swift?

[kateinoigakukun]

Hmm, swiftlint action somehow failed to fetch main branch and shows violation diagnostics for unchanged lines. Please ignore it for now.

Also IntegrationTests.testTokamakBundle is failing. carton init invokes swift package init, then rewrites some sources, and swift package init creates main.swift. So we need to remove it to avoid having both App.swift and main.swift

[kateinoigakukun]

Thanks!