Stub generation infrastructure (except for metadata generation) for KAPT+K2

This commit includes the basic Java stubs generation infrastructure and
the corresponding tests. The main entry point is called
Kapt4StubGenerator. Calls to it from production code will be added in a
separate commit.

 #KT-51982

plugins/kapt3/kapt3-compiler/testData/converter/defaultImpls.fir.txt

@@ -0,0 +1,41 @@
+@kotlin.Metadata()
+public abstract interface Intf {
+    @org.jetbrains.annotations.NotNull()
+    public static final Intf.Companion Companion = null;
+    public static final int WHITE = 2;
+
+    public default int getColor() {
+        return 0;
+    }
+
+    @kotlin.Metadata()
+    public static final class Companion {
+        private static final int BLACK = 1;
+        public static final int WHITE = 2;
+
+        private Companion() {
+            super();
+        }
+
+        public final int getBLACK() {
+            return 0;
+        }
+    }
+}
+
+////////////////////
+
+
+@kotlin.Metadata()
+public abstract interface IntfWithDefaultImpls {
+
+    public default void a() {
+    }
+}
+
+////////////////////
+
+
+@kotlin.Metadata()
+public abstract interface IntfWithoutDefaultImpls {
+}