Adjust RawPropsPropNameLength's type to account for increased number of props (facebook#39008)

Summary:
Pull Request resolved: facebook#39008

Changelog: [Internal] - Adjust RawPropsPropNameLength's type to account for increased number of props

While investigating why we needed to back out D48288752 I discovered that the root cause was that the `items_` vector in `RawProsKeyMap` was now a size greater than 255 which becomes an issue because `items_`'s indices are statically cast to `RawPropsPropNameLength` (previously alias to `uint8_t`).

This diff updates `RawPropsPropNameLength` to be an alias to `uint16_t` so the current issue is resolved as well as adding an assert to ensure (however unlikely) that this happens again.

Differential Revision: D48331909

fbshipit-source-id: e89f4c1dce36e63dc9c395eca4ff47b18b5f5e12

Author: vincentriemer

packages/react-native/ReactCommon/react/renderer/core/RawPropsKeyMap.cpp

@@ -38,6 +38,8 @@ void RawPropsKeyMap::insert(
   item.value = value;
   key.render(item.name, &item.length);
   items_.push_back(item);
+  react_native_assert(
+      items_.size() < std::numeric_limits<RawPropsPropNameLength>::max());
 }
 
 void RawPropsKeyMap::reindex() noexcept {

packages/react-native/ReactCommon/react/renderer/core/RawPropsPrimitives.h

@@ -19,7 +19,7 @@ using RawPropsValueIndex = uint8_t;
 static_assert(
     sizeof(RawPropsValueIndex) == 1,
     "RawPropsValueIndex must be one byte size.");
-using RawPropsPropNameLength = uint8_t;
+using RawPropsPropNameLength = uint16_t;
 using RawPropsPropNameHash = uint32_t;
 
 /*