Improve mapper tool accessibility

[iansan5653]

Improves the mapper tool UX to fix #61 and make it easier to use:

• Replaces the output container with an input element so we automatically get focus handling and indication
  • This element has the application role with "Input Capture" aria-roledescription. This indicates that this is not a normal text input and that keyboard events will be captured and discarded
  • Adds aria-live and aria-atomic to read out the hotkey string when it changes
  • Adds aria-labelledby and aria-describedby for semantic label & description associations to help screen reader users get oriented
  • Adds autofocus so the input is focused on load. This is usually to be avoided, but in this case there is only one reasonable place for focus to land on the page and it is expected that the page will 'work' when loaded. In addition, the label & description associations provide context to prevent this from being disorienting.
• Replaces the document-level listener with an input-level listener that ignores Tab events
• Fixes the button nested inside the clipboard-copy element (which has the button role already)
  • Also replaces the icon with a text label for enhanced accessibility, clarity, and much-simplified code
• Updates styling and layout a bit
• Moves the demo page to /demo and adds an index page to improve the discoverability of the tool

Screen.Recording.2023-09-08.at.1.33.23.PM.mov

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A few remaining known issues for future work:

• Doesn't support chords. I tried to add this in this PR but I got stuck because the current chord logic is dependent on the hotkey strings. Add support for chords to mapper tool #89
• Some browser shortcuts cannot be trapped, like Meta+t. There is nothing we can do about this.
• Screen readers (depending on settings) don't typically read puncuation names. So something like . will not be read at all. I don't know of a way to force screen readers to read this correctly without adding hidden text spelling out all of the punctuation.

[theinterned]

Thanks for doing this!