LibJS: Implement ImportCall and HostImportModuleDynamically

This allows us to load modules from scripts.
This can be dangerous as it can load arbitrary files. Because of that it
fails and throws by default. Currently, only js and JavaScriptTestRunner
enable the default hook.

This also adds tests to test-js which test module code. Because we
form a spec perspective can't "enter" a module this is the easiest way
to run tests without having to modify test-js to have special cases for
modules.
To specify modules in test-js we use the extension '.mjs' this is to
ensure the files are not executed. We do still want to lint these files
so the prettier scripts have changed to look for '.mjs' files as well.

Userland/Utilities/js.cpp

@@ -1290,6 +1290,8 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     bool syntax_highlight = !disable_syntax_highlight;
 
     vm = JS::VM::create();
+    vm->enable_default_host_import_module_dynamically_hook();
+
     // NOTE: These will print out both warnings when using something like Promise.reject().catch(...) -
     // which is, as far as I can tell, correct - a promise is created, rejected without handler, and a
     // handler then attached to it. The Node.js REPL doesn't warn in this case, so it's something we