LibWeb: Implement <input type=file> behavior

This includes punting on the actual file picker implementation all the
way out to the PageClient. It's likely that some of the real details
should be implemented somewhere closer, like the BrowsingContext or the
Page, but we'll get there.

For now, this allows https://copy.sh/v86 to load the emulation of the
preselected images all the way until it hits a call to
URL.createObjectURL.

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -30,6 +30,7 @@ static bool is_platform_object(Type const& type)
         "Document"sv,
         "DocumentType"sv,
         "EventTarget"sv,
+        "FileList"sv,
         "ImageData"sv,
         "MutationRecord"sv,
         "NamedNodeMap"sv,