feat(developer): use new message infrastructure in kmc-kmn

Moves to using the `reportMessage` callback for reporting all errors in
kmc-kmn.

This required some patching to common/web/types in order to get the new
constants in place and add support for line numbers to `CompilerEvent`,
so at the same time I refactored `compilerErrorSeverityName` into the
module.

After the refactor, coverage tests were failing because they included
the test/ folder, so cleaned that up at the same time.