WIP: add possibility to load JavaScript Modules with PluginJS

currently the PluginJS only allows for the text/javascript type. that works well for short inline scripts, or for small JavaScript files. but if one needs to build something more complex, one needs to be able to use JavaScript Modules (https://javascript.info/modules-intro) to split the code into modules, namespace/scope one's functions, etc...

this PR intends to add that functionality to Tiki by allowing the PluginJS to have a type parameter, with currently the only values allowed being text/javascript and module.