[Plugins] Introduce new API for registering compiler plugins

Original `ComponentRegistrar` exposes Project to its registration method,
  so plugins should manually register extensions to it. To prepare for
  possible unbound compiler from Project API in future new  `K2ComponentRegistrar`
  introduced which provides registration method without Project at all

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/extensions/FirExtensionRegistrar.kt

@@ -20,10 +20,6 @@ abstract class FirExtensionRegistrar : FirExtensionRegistrarAdapter() {
             return FirExtensionRegistrarAdapter.getInstances(project) as List<FirExtensionRegistrar>
         }
 
-        fun registerExtension(project: Project, extension: FirExtensionRegistrar) {
-            FirExtensionRegistrarAdapter.registerExtension(project, extension)
-        }
-
         internal val AVAILABLE_EXTENSIONS = listOf(
             FirStatusTransformerExtension::class,
             FirDeclarationGenerationExtension::class,