Improve toolbox behaviour for keyboard and screen reader users

[microbit-robert]

This PR:

• Uses a flat tree structure for the toolbox
  • This fixes the tree item count for screen readers (these tree items are now always in the DOM but visually hidden)
  • As there are no controls to expand or collapse the sub-categories (typically left/right arrow), we don't need levels of nesting or aria-levels. Categories are simply shown visually when navigated to
• Aligns toolbox behaviour with Blockly
  • The selected category is remembered when navigating back to the toolbox. The previous behaviour hindered users when they needed to add more than one block from the same category.
• Enter / Space on a category no longer toggles the flyout open / closed
  • This was observed to be a confusing behaviour for screen reader users
  • Enter / Space still opens the extensions overlay
  • Mouse clicks still toggle the flyout open / closed
• Ensures that using the down arrow from the search input always focuses the first item in the toolbox rather than re-focusing the previously selected category - the latter is what the Tab key does
• Renders the LED category in uppercase to improve screen reader output
  • When capitalised the screen reader outputs "L E D", otherwise "led"
  • LED is typically uppercase for translations, but this is not the case for the English text
• Adds an id to the toolbox to ensure that code related to Blockly's focus manager runs (see fix: Short-circuit node lookups for missing IDs RaspberryPiFoundation/blockly#9174)