LibJS: Follow the spec with storing im- and export entries

Because we can have arbitrary in- and export names with strings we can
have '*' and '' which means using '*' as an indicating namespace imports
failed / behaved incorrectly for string imports '*'.
We now use more specific types to indicate these special states instead
of these 'magic' string values.

Do note that 'default' is not actually a magic string value but one
specified by the spec. And you can in fact export the default value by
doing: `export { 1 as default }`.

Userland/Libraries/LibJS/Tests/modules/string-import-namespace-indirect.mjs

@@ -0,0 +1,6 @@
+import * as indirectNs from "./default-and-star-export-indirect.mjs";
+
+export const passed =
+    indirectNs["*"] === "starExportValue" &&
+    indirectNs[""] === "empty" &&
+    indirectNs.default === undefined;