Belongs to d382a3cccf, but I authored that
on another machine.
Making sure vscode uses mypy from the environment is very important now,
because some of these errors get triggered in different ways depending
on the version.
This makes the tests more readable and easier to browse.
Making sure the tests always run fine, no matter from which directory
they're started is a little detail we should either test for or
document.
Using untyped dicts was not a safe way to pass arguments to error
message rendering. Also, having one giant match statement wouldn't scale
well. Using function pointers and kwargs, mypy can now properly check
whether all required arguments are passed to DiskoMessage().
As a bonus, it's now easy to use the "Go to definition" LSP command for
error codes, which wasn't possible before.
