Add ultrabold pairs for font weight (facebook#24948)

Summary:
Add `ultrabold` map of font weight, if not, `ultrabold` would map to `bold`.
Fixes facebook#23512.

## Changelog

[iOS] [Fixed] - Add ultrabold pairs for font weight
Pull Request resolved: facebook#24948

Differential Revision: D15575568

Pulled By: cpojer

fbshipit-source-id: 5d1d6a033c166d91a330526ba8996ac0416f3887

React/Views/RCTFont.mm

@@ -32,6 +32,7 @@ static RCTFontWeight weightOfFont(UIFont *font)
       @"semibold",
       @"demibold",
       @"extrabold",
+      @"ultrabold",
       @"bold",
       @"heavy",
       @"black"
@@ -46,6 +47,7 @@ static RCTFontWeight weightOfFont(UIFont *font)
       @(UIFontWeightSemibold),
       @(UIFontWeightSemibold),
       @(UIFontWeightHeavy),
+      @(UIFontWeightHeavy),
       @(UIFontWeightBold),
       @(UIFontWeightHeavy),
       @(UIFontWeightBlack)