Added support for class diagram filtering based on C++20 modules (#195)

tests/t00070/src/common.cppm

@@ -0,0 +1,11 @@
+export module t00070;
+export import t00070.lib1;
+export import t00070.lib2;
+
+export namespace clanguml::t00070 {
+class A {
+    int get() { return a; }
+
+    int a;
+};
+}