feat(GlFormTextarea): add support for character count

mentioned in commit ecd279a8

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!312 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!150344 (merged)

mentioned in merge request gitlab!150653 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3826 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3827 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!33 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!34 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!478 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!486 (closed)