Implement URLSearchParams size getter

In preparation for whatwg/url#734

(this shouldn't land until that spec change lands)

Author: jasnell

src/workerd/api/url-standard.h

@@ -123,7 +123,10 @@ class URLSearchParams: public jsg::Object {
 
   jsg::UsvString toString();
 
+  inline uint getSize() { return list.size(); }
+
   JSG_RESOURCE_TYPE(URLSearchParams) {
+    JSG_READONLY_PROTOTYPE_PROPERTY(size, getSize);
     JSG_METHOD(append);
     JSG_METHOD_NAMED(delete, delete_);
     JSG_METHOD(get);

src/workerd/api/url.h

@@ -170,7 +170,10 @@ class URLSearchParams: public jsg::Object {
 
   kj::String toString();
 
+  inline uint getSize() { return url->query.size(); }
+
   JSG_RESOURCE_TYPE(URLSearchParams, CompatibilityFlags::Reader flags) {
+    JSG_READONLY_PROTOTYPE_PROPERTY(size, getSize);
     JSG_METHOD(append);
     JSG_METHOD_NAMED(delete, delete_);
     JSG_METHOD(get);