Implement extra keyboard controls

[corymharper]

This pull request implements all of the pertinent optional keyboard controls listed on this page: https://www.w3.org/TR/wai-aria-practices/#keyboard-interaction-12

Those include:

Space: It will now activate menu items and close the menu after activation
Printable characters (not including those that have some other function): Will move the focus to the first menu item whose label starts with the corresponding character

Closes #269

[codecov]

Codecov Report

Merging #272 (4667844) into master (0ddd2f2) will increase coverage by 0.70%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
+ Coverage   91.76%   92.47%   +0.70%     
==========================================
  Files           1        1              
  Lines          85       93       +8     
  Branches       24       29       +5     
==========================================
+ Hits           78       86       +8     
  Misses          7        7              

Impacted Files	Coverage Δ
src/use-dropdown-menu.ts	92.47% <100.00%> (+0.70%)	⬆️

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[WesCossick]

Can you also update the PR description to describe in a little more detail what these optional keys do? So people don't have to dig through the documentation.

Requested changes made