Set small margin on the textarea element in textarea component

This element defaults to a 30px margin which is the same as the outer
form-group element and thus it collapses into it. I intend to place
elements inside this form group after the textarea and these look
visually better with a small 5px margin.

app/assets/stylesheets/govuk_publishing_components/components/_textarea.scss

@@ -1,3 +1,7 @@
 @import "helpers/govuk-frontend-settings";
 @import "govuk-frontend/components/textarea/textarea";
 @import "govuk-frontend/objects/form-group";
+
+.gem-c-textarea {
+  margin-bottom: govuk-spacing(1);
+}