Allow `\clear` to clean Duo Chat list along with `\clean`
### Problem
I find it hard to remember whether the command to clean the Duo Chat screen is `/clear` or `/clean`. I essentially have a 50/50 shot to get it right.
Others also have this problem:
* https://gitlab.slack.com/archives/C06D5C70MD2/p1709834243236389
* https://gitlab.com/gitlab-org/gitlab/-/issues/438463#note_1785333518
* https://gitlab.com/gitlab-org/gitlab/-/issues/430124#note_1723494660
* https://gitlab.com/gitlab-org/gitlab/-/issues/430124#note_1653573302
* https://gitlab.com/orange-trial/gitlab-duo-workshop/richard-just-a-web-app/-/issues/1#note_1736431430
Also, if there is a trailing space after `/clean` it is interpreted as a question rather than a command:
* https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/issues/1281
### Proposal
1. Add a new command `/clear` that behaves like the current `/clean` command in Duo Chat.
2. Make `/clear` the standard command and show it in the command options in VSCode. Update the documentation to show just `/clear`
3. Ignore preceding and trailing spaces on both commands
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