LibWeb: Add Exposed attribute and IDL spec links where missing

The intent is to use these to autogenerate prototype declarations for
Window and WorkerGlobalScope classes.

And the spec links are just nice to have :^)

Userland/Libraries/LibWeb/HTML/HTMLSelectElement.idl

@@ -1,6 +1,8 @@
 #import <HTML/HTMLElement.idl>
 #import <HTML/HTMLOptionsCollection.idl>
 
+// https://html.spec.whatwg.org/multipage/form-elements.html#htmlselectelement
+[Exposed=Window]
 interface HTMLSelectElement : HTMLElement {
 
     [Reflect] attribute boolean disabled;