Add 'props.lineBreakMode' to Text

Summary:I've tested this manually, but I'm not sure how to write a test for this. Hopefully someone can help out there. The least I could do is provide a starting point for a PR to be accepted.

Additionally, I've renamed the existing `NSLineBreakMode` enum converter (inside `RCTConvert`) to use dashes in the names instead of camelcase (eg: `word-wrapping` instead of `wordWrapping`).

Fixes #6338
Closes https://github.com/facebook/react-native/pull/6339

Differential Revision: D3052391

Pulled By: nicklockwood

fb-gh-sync-id: 1536dfc139d7995095e9ee9d5f13ca86f90783c5
shipit-source-id: 1536dfc139d7995095e9ee9d5f13ca86f90783c5

Libraries/Text/TextStylePropTypes.js

@@ -40,6 +40,17 @@ var TextStylePropTypes = Object.assign(Object.create(ViewStylePropTypes), {
    */
   letterSpacing: ReactPropTypes.number,
   lineHeight: ReactPropTypes.number,
+  /**
+   * @platform ios
+   */
+  lineBreakMode: ReactPropTypes.oneOf([
+    'clipping',
+    'word-wrapping',
+    'char-wrapping',
+    'truncating-head',
+    'truncating-middle',
+    'truncating-tail',
+  ]),
   /**
    * Specifies text alignment. The value 'justify' is only supported on iOS.
    */