feat(DuoChat): Emit chat-slash event on slash commands opened

mentioned in commit 9e151879

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!172936 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4179 (merged)

mentioned in merge request duo-ui!25 (merged)

mentioned in merge request gitlab!173375 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)