Support gitlab pages target in dendron publish command

[rlh1994]

Please select if your request is either something new or an enhancement

• [x] Enhancement of an existing Feature.
• [ ] Request of a new feature.

Please select the area your request applies to. (Multiple selections are Possible. You can leave blank if you're not sure.)

• [ ] Workspace - VSCode workspace, vaults, Intellisense/autocomplete, Dendron settings
• [ ] Lookup - Dendron's Lookup Command
• [ ] Views - Dendron Preview, Tree View, Side Panels in the UI
• [ ] Schema - Dendron Schemas
• [ ] Pod - Data import from / export to Dendron
• [x] Publish - External Site Publish
• [ ] Markdown - Markdown features, such as syntax support and features

Is your feature request related to a problem? Please describe

Currently it is possible to set the output to the correct folder for a github pages site using the --target flag in the dendron publish export command, but not gitlab pages.

Describe the solution you'd like

An option in this flag for gitlab.

Describe alternatives you've considered

Additional context

I'm pretty sure it should just be adding a new branch to the switch statement in the below function that accepts gitlab and instead of docs the folder is instead public, and wherever converts from flags to the calls, but I don't know how to add all the tests and everything else that would be required to do this contribution.

https://github.com/dendronhq/dendron/blob/4132cd35eb76bc2a19f5a8e0309cee755e9bf42e/packages/dendron-cli/src/commands/publishCLICommand.ts#L286-L327

[hikchoi]

Thank you for the suggestion!