feat: add a character counter to gl-form-textarea

What does this MR do?

Provides a recommended character count option and a maximum character count option.

The recommended character count is always displayed, counting down to 0, formatted by the characterCountText prop. It remains gray even when the difference is 0. Once the threshold is crossed, the count is displayed as grey, formatted by the characterCountOverLimitText. This is set by using the countType of 'recommended'.

The maximum character count is always displayed, counting down to 0, formatted by the characterCountText prop. It remains grey if the remaining count is 0. Once the threshold has been crossed, the count is displayed as a negative value with danger text and formatted by the characterCountOverLimitText. This is set by using the countType of 'max', and is the default choice.

This is only enabled if count is truthy, as a max or recommended count of 0 don't really make sense.

NO DEFAULT FORMATTED TEXT IS PROVIDED FOR characterCountText OR characterCountOverLimitText. THIS IS BECAUSE THERE ARE NO TRANSLATIONS AVAILABILE IN THIS PROJECT.

The recommended values are below:

For max type

• characterCountText: '%{count} characters left'
• characterCountOverLimitText: '%{count} characters over limit'

For recommended type

• characterCountText: '%{count} recommended characters left'
• characterCountTextOverLimit: '%{count} over recommended length'

Closes Add character count support to GlFormTextarea (#1243 - closed)

Does this MR meet the acceptance criteria?

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Accessibility

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• All actions and functionality can be done with a keyboard.
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