Right and Left Arrow Event Handling

Summary:
Exposes right and left keyboard events, as suggested in [this issue](#219) by hellendag.

This will greatly simplify some of our arrow handling at [Artsy](https://github.com/artsy/positron/blob/master/client/apps/edit/components/content2/sections/text/index.coffee#L121).

The test suite is passing locally.
Closes #1384

Differential Revision: D5954003

fbshipit-source-id: 90c3a7d6bb5070c455f56d60ccd64c5ad136fe59

docs/APIReference-Editor.md

@@ -245,10 +245,21 @@ onTab?: (e: SyntheticKeyboardEvent) => void
 onUpArrow?: (e: SyntheticKeyboardEvent) => void
 ```
 
+#### onRightArrow
+```
+onRightArrow?: (e: SyntheticKeyboardEvent) => void
+```
+
 #### onDownArrow
 ```
 onDownArrow?: (e: SyntheticKeyboardEvent) => void
 ```
+
+#### onLeftArrow
+```
+onLeftArrow?: (e: SyntheticKeyboardEvent) => void
+```
+
 ### Mouse events
 
 ### onFocus

src/component/base/DraftEditorProps.js

@@ -156,7 +156,9 @@ export type DraftEditorProps = {
   onEscape?: (e: SyntheticKeyboardEvent<>) => void,
   onTab?: (e: SyntheticKeyboardEvent<>) => void,
   onUpArrow?: (e: SyntheticKeyboardEvent<>) => void,
+  onRightArrow?: (e: SyntheticKeyboardEvent<>) => void,
   onDownArrow?: (e: SyntheticKeyboardEvent<>) => void,
+  onLeftArrow?: (e: SyntheticKeyboardEvent<>) => void,
 
   onBlur?: (e: SyntheticEvent<>) => void,
   onFocus?: (e: SyntheticEvent<>) => void,

src/component/handlers/edit/editOnKeyDown.js

@@ -109,9 +109,15 @@ function editOnKeyDown(editor: DraftEditor, e: SyntheticKeyboardEvent<>): void {
     case Keys.UP:
       editor.props.onUpArrow && editor.props.onUpArrow(e);
       return;
+    case Keys.RIGHT:
+      editor.props.onRightArrow && editor.props.onRightArrow(e);
+      return;
     case Keys.DOWN:
       editor.props.onDownArrow && editor.props.onDownArrow(e);
       return;
+    case Keys.LEFT:
+      editor.props.onLeftArrow && editor.props.onLeftArrow(e);
+      return;
     case Keys.SPACE:
       // Handling for OSX where option + space scrolls.
       if (isChrome && isOptionKeyCommand(e)) {