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/maxErrorCount.fir.txt

@@ -0,0 +1,12 @@
+@kotlin.Metadata()
+public final class Test {
+
+    public Test() {
+        super();
+    }
+
+    public final void a(@org.jetbrains.annotations.NotNull()
+    ABC a, @org.jetbrains.annotations.NotNull()
+    BCD b) {
+    }
+}