Preparation for moving to platform specific extension

VS Code recently added support for platform specific extensions. This can be used instead of downloading binaries in the extension itself, and thus we get rid of the problems this causes.

However, doing that means that the paths to the compiler and to the soiheader change. This is a problem, because these paths were stored in the soiworkspace.json file. To solve this, I moved the configuration to the VS Code configuration API. Settings from old soiworkspace.json files will not be automatically migrated to the new location, and in the new place, configurations are only stored if the user explicitly changes them.

I also rewrote the user program runner in JavaScript using CustomExecution. This means that once we move to platform specific extensions, we can drop the Go helper entirely.

For more details, see the commit messages.