Xindi/keyboard control

[xindixu]

Exactly the same as PR in original repo

Changes

Added keyboard control:

Category anchors:

• When focusing on any emoji, Tab to change focus to the first category anchor
• When a category is selected, Arrow Down to go to the first item in the category
• This also means that when focusing on any emoji, Tab can no longer used to go to the next emoji
• Same applies to search
  

Emoji:

• Arrow Up, Arrow Down, Arrow Left, Arrow Right to go to emojis on top/down/left/right to the current emoji
• Able to jump to next/previous category if at the last line/first row

Style:

• emoji focus style now should look the same as emoji on hover style

Markup:

• Change emoji layout to table/cell with a11y roles

References:

• This PR is largely inspired by Better keyboard navigation/selection missive/emoji-mart#348. Note that some of the features mentioned are not implemented or implemented differently.
• https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/Grid_Role
• Slack emoji picker