Bringing back an old pull request from Michelle to add keyboard accessibility: #269.

From Michelle's PR:

• Added "startKeyboardDrag", which uses new util functions to calculate the start and end points differently from mouse events.
• Moved chunks of "startMouseDrag" out into separate functions so that keyboard drag can share the code: this.startDragging(), this.drag();, this.stopDragging();
• Consumers are able to optionally add a text for aria-label via @input().

Additional features:

• Added role="slider" and made the dragger/gutter tab-able.
• Added aria-orientation, aria-valuemin, aria-valuemax, aria-valuenow, aria-valuetext.
• Added PageUp and PageDown support. Arrow keys does a step offset of 50. PageUp and PageDown does a step offset of 500.
• Added util functions to help test keyboard functionality in the e2e tests. Changed "moveGutter" into "moveGutterByMouse" and introduced "moveGutterByKeyboard", "checkGutterAriaLabel", "checkGuttersAriaValueTexts"

Issues discovered while developing:

• Tab order of the gutters is not top-down left-right
• In screenreader mode, displayed areas with a size of "*" are not supported because size is null in the displayed area.

Keyboard support:

Screen reader: