move FormData to xhr folder

src/browser/env.zig

@@ -35,7 +35,7 @@ const WebApis = struct {
         @import("storage/storage.zig").Interfaces,
         @import("url/url.zig").Interfaces,
         @import("xhr/xhr.zig").Interfaces,
-        @import("url/form_data.zig").Interfaces,
+        @import("xhr/form_data.zig").Interfaces,
         @import("xmlserializer/xmlserializer.zig").Interfaces,
     });
 };

src/browser/xhr/form_data.zig

@@ -46,6 +46,8 @@ pub const Interfaces = .{
 // This is much easier to do with an ArrayList than a HashMap, especially given
 // that the FormData could be mutated while iterating.
 // The downside is that most of the normal operations are O(N).
+
+// https://xhr.spec.whatwg.org/#interface-formdata
 pub const FormData = struct {
     entries: std.ArrayListUnmanaged(Entry),