* Update the project to Swift 5.4
- Splash now uses the Swift 5.4 toolchain.
- Since Swift 5.4 now enables automatic test discovery on all supported
platforms, all of the infrastructure that was previously used to maintain
Linux unit testing compatibility has now been removed.
* Remove LinuxMain.swift
This patch makes `HTMLOutputFormat` correctly escape all < and > characters
that occur in source code. Otherwise, a web browser rendering the resulting
HTML will interpret those characters as tags.