Implement WAI Window Splitter guidance

[KenanYusuf]

Fixes #1

• [x] Left Arrow: Moves a vertical splitter to the left.
• [x] Right Arrow: Moves a vertical splitter to the right.
• [x] Up Arrow: Moves a horizontal splitter up.
• [x] Down Arrow: Moves a horizontal splitter down.
• [x] Home (Optional): Moves splitter to the position that gives the primary pane its smallest allowed size. This may completely collapse the primary pane.
• [x] End (Optional): Moves splitter to the position that gives the primary pane its largest allowed size. This may completely collapse the secondary pane.
• [x] Enter: If the primary pane is not collapsed, collapses the pane. If the pane is collapsed, restores the splitter to its previous position.
• [ ] F6 (Optional): Cycle through window panes. Not going to implement this recommendation for now as it is optional and overwriting function keys seems a contentious issue.
• [x] The element that serves as the focusable splitter has role separator.
• [x] The separator element has the aria-valuenow property set to a decimal value representing the current position of the separator.
• [x] The separator element has the aria-valuemin property set to a decimal value that represents the position where the primary pane has its minimum size. This is typically 0.
• [x] The separator element has the aria-valuemax property set to a decimal value that represents the position where the primary pane has its maximum size. This is typically 100.
• [x] If the primary pane has a visible label, it is referenced by aria-labelledby on the separator element. Otherwise, the separator element has a label provided by aria-label. Added an example the demo app and README, as these attributes can be set when implemented.
• [x] The separator element has aria-controls referring to the primary pane.