fix: extend forward / backward (#2314)

#### Is this adding or improving a _feature_ or fixing a _bug_?

_bug_

#### What's the new behavior?

Fix behavior for <kbd>shift + left</kbd> and <kbd>shift + right</kbd>. Now they will correctly move forward / backward.

#### How does this change work?

turns out that the hotkey `'left'` would also pick up with a modifier key `'shift+left'` so checking for `Hotkeys.isExtendForward()` needs to come before `Hotkeys.isMoveForward()` and it needs to short circuit with `return true`. Likewise for backward.

#### Have you checked that...?

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* [x] The new code matches the existing patterns and styles.
* [x] The tests pass with `yarn test`.
* [x] The linter passes with `yarn lint`. (Fix errors with `yarn prettier`.)
* [x] The relevant examples still work. (Run examples with `yarn watch`.)

#### Does this fix any issues or need any specific reviewers?

Fixes: #2307 
Reviewers: @ianstormtaylor

packages/slate-hotkeys/package.json

@@ -13,7 +13,7 @@
     "lib/"
   ],
   "dependencies": {
-    "is-hotkey": "^0.1.3",
+    "is-hotkey": "0.1.4",
     "slate-dev-environment": "^0.2.0"
   },
   "scripts": {