Changes the character count to inject a new element that is
live updated, instead of overwriting the content of the initial
HTML hint.

This is intended to resolve an issue where screen readers are
reading out all elements referenced in aria-describedby for
each keystroke, and not just the updated counter element.

#2539

Debounces the character counter update until 250 milliseconds after the user has stopped typing. This helps prevent multiple rapid-fire updates being queued up by screen readers and read out afterwards, and prevents "stuttering" by screen readers which attempt to read out the updated
counter and the user's input simultaneously.

The handleFocus method's bugfix for Dragon Naturally Speaking now runs a check to see when the last user input was provided, and will not update the counter if the user has recently typed anything. This prevents the DNS fix from causing the same queuing and stuttering behaviour.

This also fixes a newly identified bug where the keyup, focus and blur event listeners were all being bound twice due to the sync method being called both on script initialisation and on pageshow/DOMContentLoaded events. The sync method has been removed and this now calls
updateCountMessage directly, which is the part that actually requires syncronisation.

Adds wait time to tests that involve changing the value of the character counter's textarea, so that the 250ms debounce timer can complete the update before the result is tested for.

Splits the character counter into two:
- A visible counter, that updates immediately, and is hidden to screen readers.
- An screen reader counter, that updates only after the user has stopped typing for one second, and is visually hidden.

Some issues were being caused by there being two updates that happened after the user had stopped typing: One being the check that exists for voice dictation users
(which is debounced so that it doesn't run the check if a user is typing), the other being the debounched update from the keyboard event handling.

These updates could happen very close to one another, causing some screen readers to repeat the updated message or to stop announcing the updates altogether.