AK+Everywhere: Add and use static APIs for LexicalPath

The LexicalPath instance methods dirname(), basename(), title() and
extension() will be changed to return StringView const& in a further
commit. Due to this, users creating temporary LexicalPath objects just
to call one of those getters will recieve a StringView const& pointing
to a possible freed buffer.

To avoid this, static methods for those APIs have been added, which will
return a String by value to avoid those problems. All cases where
temporary LexicalPath objects have been used as described above haven
been changed to use the static APIs.

Userland/Libraries/LibTest/JavaScriptTestRunnerMain.cpp

@@ -56,7 +56,7 @@ int main(int argc, char** argv)
 {
     g_test_argc = argc;
     g_test_argv = argv;
-    auto program_name = LexicalPath { argv[0] }.basename();
+    auto program_name = LexicalPath::basename(argv[0]);
     g_program_name = program_name;
 
     struct sigaction act;