Improve keyboard support on completion overlay - PageUp / PageDown navigation, arrow nav + more

* Completion overlay menu should allow for PageUp / PageDown navigation

* An up arrow key at the top of the list should move to the bottom of the list (like  cmd palette)

* Similarly, a down arrow at the bottom of the list should move to the top of the list

* For completions that start with or in any other way use the $ character (for instance, $('#select DOM element') for jQuery) the completion menu does not show automatically -- you have to hit $, then TAB to show the available completions

* Once the completion menu is open, typing should filter on both the tabTrigger and the name -- presently, only the tabTrigger is used.  This can cause some unexpected results, as this overlay doesn't quite behave the same as the Ctrl + Shift + P command palette overlay.