`PluginProblemReporter` is not registered as application service

As seen in https://github.com/google/ksp/pull/3049 and https://github.com/google/ksp/pull/3138, KSP can crash if `PluginProblemReporter` is transitively used. The fix should be to register it as an application service such that `PluginProblemRepoerter.getInstance()` returns a valid instance.