Implement Duo Chat slash command UI on GitLab
## Context
Slash commands enable the user to communicate their intent to Duo Chat in an efficient way. They are also a way for the user to understand the functionalities that Duo Chat has.
A slash command UI exists in gitlab-ui and the IDEs but not GitLab.
| IDE | [gitlab-ui](https://gitlab-org.gitlab.io/gitlab-ui/?path=/docs/experimental-duo-chat-duo-chat--docs#interactive) | GitLab |
| ------ | ------ | ------ |
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## Proposal of iterations
Here's a rough suggestion of iterations:
| Iteration | Behavior | Related issues |
| ------ | ------ | ------ |
| 1 | Menu only shows commands that are shown on every page, e.g. `/reset` command, when the user selects `/reset` it is immediately sent (as opposed filled in the text input) | https://gitlab.com/gitlab-org/gitlab/-/issues/470703 |
| 1a | (depending on timing) Add new commands that are shown on every page, e.g. `/help` | https://gitlab.com/gitlab-org/gitlab/-/issues/462122 |
| 1b | (depending on timing) Refactor commands on every page to match new standards | https://gitlab.com/gitlab-org/gitlab/-/issues/470818 |
| 2 | Figure out the technical implementation plan to support that different pages offer different slash commands | https://gitlab.com/gitlab-org/gitlab/-/issues/470704 |
| 3 and future | Add additional commands to the menu UI | https://gitlab.com/groups/gitlab-org/-/epics/14493, https://gitlab.com/groups/gitlab-org/-/epics/14404 |
epic