move default modules from /modules/default to /defaultmodules (#4019)

Since the project's inception, I've missed a clear separation between
default and third-party modules.

This increases complexity within the project (exclude `modules`, but not
`modules/default`), but the mixed use is particularly problematic in
Docker setups.

Therefore, with this pull request, I'm moving the default modules to a
different directory.

~~I've chosen `default/modules`, but I'm not bothered about it;
`defaultmodules` or something similar would work just as well.~~

Changed to `defaultmodules`.

Let me know if there's a majority in favor of this change.

js/defaults.js

@@ -20,6 +20,7 @@ const defaults = {
 	zoom: 1,
 	customCss: "css/custom.css",
 	foreignModulesDir: "modules",
+	defaultModulesDir: "defaultmodules",
 	// httpHeaders used by helmet, see https://helmetjs.github.io/. You can add other/more object values by overriding this in config.js,
 	// e.g. you need to add `frameguard: false` for embedding MagicMirror in another website, see https://github.com/MagicMirrorOrg/MagicMirror/issues/2847
 	httpHeaders: { contentSecurityPolicy: false, crossOriginOpenerPolicy: false, crossOriginEmbedderPolicy: false, crossOriginResourcePolicy: false, originAgentCluster: false },