Add minPointers prop to ScrollView component

[naquilini]

Introduction

ScrollView component currently does not support the activation of scroll gesture only when 2 fingers (or more) moves on the screen. The ScrollView (if scroll enabled) always activates when a single gingers moves on the screen.

Details

The underlying native components could already support that.
For instance, in iOS a change like the following in the native code could add the support to minPointers:

diff --git a/node_modules/react-native/React/Views/ScrollView/RCTScrollView.h b/node_modules/react-native/React/Views/ScrollView/RCTScrollView.h
index 10ef46a..a7f74bb 100644
--- a/node_modules/react-native/React/Views/ScrollView/RCTScrollView.h
+++ b/node_modules/react-native/React/Views/ScrollView/RCTScrollView.h
@@ -48,6 +48,7 @@
 @property (nonatomic, assign) BOOL snapToEnd;
 @property (nonatomic, copy) NSString *snapToAlignment;
 @property (nonatomic, assign) BOOL inverted;
+@property (nonatomic, assign) int minPointers;
 /** Focus area of newly-activated text input relative to the window to compare against UIKeyboardFrameBegin/End */
 @property (nonatomic, assign) CGRect firstResponderFocus;
 /** newly-activated text input outside of the scroll view */
diff --git a/node_modules/react-native/React/Views/ScrollView/RCTScrollView.m b/node_modules/react-native/React/Views/ScrollView/RCTScrollView.m
index 6f41b5c..8612ba6 100644
--- a/node_modules/react-native/React/Views/ScrollView/RCTScrollView.m
+++ b/node_modules/react-native/React/Views/ScrollView/RCTScrollView.m
@@ -477,6 +477,9 @@ - (void)didSetProps:(NSArray<NSString *> *)changedProps
   if ([changedProps containsObject:@"contentSize"]) {
     [self updateContentSizeIfNeeded];
   }
+  if ([changedProps containsObject:@"minPointers"]) {
+    _scrollView.panGestureRecognizer.minimumNumberOfTouches = self.minPointers;
+  }
 }
 
 - (BOOL)centerContent
diff --git a/node_modules/react-native/React/Views/ScrollView/RCTScrollViewManager.m b/node_modules/react-native/React/Views/ScrollView/RCTScrollViewManager.m
index cd1e7eb..607d498 100644
--- a/node_modules/react-native/React/Views/ScrollView/RCTScrollViewManager.m
+++ b/node_modules/react-native/React/Views/ScrollView/RCTScrollViewManager.m
@@ -101,6 +101,7 @@ - (UIView *)view
 RCT_EXPORT_VIEW_PROPERTY(inverted, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(automaticallyAdjustsScrollIndicatorInsets, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(contentInsetAdjustmentBehavior, UIScrollViewContentInsetAdjustmentBehavior)
+RCT_EXPORT_VIEW_PROPERTY(minPointers, int)
 
 // overflow is used both in css-layout as well as by react-native. In css-layout
 // we always want to treat overflow as scroll but depending on what the overflow
```

### Discussion points

- was there a specific reason why a `minPointers` prop was not implemented?
- is there another way to achieve the desired response to user touch gestures to achieve the desired effect?