Checkbox with revealed text area

[MagdalenaLarge]

As Software Developer
I want to be able to add checkbox with the revealed text area
So that users responding "Other" can provide the legally required justification.

Description
This request came from the Community discussion board - Checkbox

Due to the tightly regulated format of the form, whenever respondent selects "Other" option, they need to provide up to 700 words (3500 characters) justification. However this field should be editable - users can adjust the number of characters as per their individual needs.
The characters count needs to be added too. It will be counting down 'characters remaining', like all other existing design system components.
Text area will need to be able to accommodate the text size. User expects the text area to expand horizontally to the limit of it's containing parent/grid element + obey existing Nunjucks size attributes for textarea. If the content then gets too big for textarea to display then should only display a vertical scrollbar.

Acceptance Criteria
Scenario: Populate textarea when "other" checkbox is selected
Given the user is on the form page
When the user selects the checkbox called "other"
Then the textarea is revealed

Scenario: Limit the number of characters user can add
Given the user is on the form page
And the textarea is populated
When the user tries to add characters
Then the user can add a maximum number of characters as stated under the textarea

Scenario: Trigger remaining character count on any input
Given the user is on the form page
And the textarea is populated
When the user inputs characters
Then the remaining character count is triggered

Scenario: Allow user to input and delete characters
Given the user is on the form page
And the textarea is populated
When the user inputs characters
And the user deletes characters
Then the textarea reflects the input and deletion of characters

Scenario: Adjust textarea size based on the number of characters
Given the user is on the form page
And the textarea is populated
When the user inputs characters
Then the textarea size expands horizontally to the limit of it's containing parent/grid element

Scenario: Scroll bar within textarea size based on the number of characters
Given the user is on the form page
And the textarea is populated
When the user inputs large number of characters
Then the textarea will display vertical scrollbar

Visualisation/Input from UCD
This is the visualisation of the requirement provided by the requestor

Linked User stories/epics
Currently we have similar functionality in the
1/ Checkbox error message where user can select other and it reveals textarea.
2/ Radios with reveled textarea

Technical details (optional)
Delivery expected by end of November