Improved Focusing, $el as a button

[sagalbot]

Updates the root $el to be a focusable button instead of a div. Allows for separating focusing of the component from opening the dropdown.

Closes #1054
Closes #944
Related #872

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• swap root $el for button
• implement button keypress handler
• • implementation should mimic native <select>
• • space should open dropdown, but not enter the character to search
• • return does nothing
• • modifier keys don't toggle the dropdown
• • up/down toggles dropdown, doesn't adjust typeahead index
• • left/right selects the next option in either direction
• • • left/right doesn't loop the list, stops at first and last
• update dropdown tests