[FIR] Make FirExtensionRegistrar a proper extension point

After that commit it's allowed to register any FIR plugin using
  existing extension infrastructure (`-Xplugin` CLI argument,
  plugin.xml for IDE plugins, etc)

compiler/tests-common/tests/org/jetbrains/kotlin/fir/AbstractFirBaseDiagnosticsTest.kt

@@ -98,6 +98,7 @@ abstract class AbstractFirBaseDiagnosticsTest : BaseDiagnosticsTest() {
                 librariesScope = PsiBasedProjectFileSearchScope(allProjectScope),
                 lookupTracker = null,
                 providerAndScopeForIncrementalCompilation = null,
+                extensionRegistrars = emptyList()
             ) {
                 configureSession()
                 getFirExtensions()?.let {