Support constructing UIEdgeInsets with single value

Summary:
This is only required in the old renderer, as in Fabric the parsing behaviour is shared across platforms (https://github.com/facebook/react-native/blob/main/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm#L264). JS changes are following in D33453327.

Changelog: [iOS][Fixed] Enable hitSlop to be set using a single number.

Reviewed By: philIip

Differential Revision: D33453326

fbshipit-source-id: f15a4fd1d26dcac6f40b064be1a8266051b6aedf

React/Base/RCTConvert.m

@@ -554,7 +554,24 @@ +(type)type : (id)json                                      \
 RCT_CGSTRUCT_CONVERTER(CGPoint, (@[ @"x", @"y" ]))
 RCT_CGSTRUCT_CONVERTER(CGSize, (@[ @"width", @"height" ]))
 RCT_CGSTRUCT_CONVERTER(CGRect, (@[ @"x", @"y", @"width", @"height" ]))
-RCT_CGSTRUCT_CONVERTER(UIEdgeInsets, (@[ @"top", @"left", @"bottom", @"right" ]))
+
++ (UIEdgeInsets)UIEdgeInsets:(id)json
+{
+  static NSArray *fields;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    fields = @[ @"top", @"left", @"bottom", @"right" ];
+  });
+
+  if ([json isKindOfClass:[NSNumber class]]) {
+    CGFloat value = [json doubleValue];
+    return UIEdgeInsetsMake(value, value, value, value);
+  } else {
+    UIEdgeInsets result;
+    convertCGStruct("UIEdgeInsets", fields, (CGFloat *)&result, json);
+    return result;
+  }
+}
 
 RCT_ENUM_CONVERTER(
     CGLineJoin,