#### 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...?
<!--
Please run through this checklist for your pull request:
-->
* [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
* fix: don't check for adjacent void with modified move
This was causing a problem where when the current text was followed by a void node using a modifier key to move forward would force the selection to creep forward a character at a time.
With this change, now the modifier will move as expected, but will jump over void nodes. This is not ideal, but seems like a behavior that will be slightly better than the current one.
* fix: modified key movement.
Use TextUtils.getWordOffsetForward and TextUtils.getWordOffsetBackward to move around by words.
The idea now is that if you move forward or backward, it is completely controlled by slate instead of trying to rely on a combination of browser behavior and slate trying to stop the browser from doing something wrong. This makes things quite a bit more intuitive in the implementation, and gives us a bit more control.
* tests: a whole bunch of selection movement tests.
* tests: more movement tests.
* Extract hotkey and environment detection into separate packages
Overriding default behavior in plugins can be hard, because you have
to match all of the keybindings that Slate uses. By exporting hotkeys
as its own package, both core Slate plugins and custom plugins can use
the same key detection logic.
* Rename Hotkeys.* to Hotkeys.is*
