add mkdir function to nextcloudcmd

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## Feature request

**Which Nextcloud Version are you currently using:** (see administration page)

- Server: [Nextcloud Hub 10] (31.0.6)
- Client: Nextcloud version 3.16.6daily

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

- I need to create a folder on a nextcloud server in a script.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

- a --mkdir /path/to/new/dir option for nextcloudcmd

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

- make a temporary dir ("/tmp") locally in which there is the dir ("/tmp/a") I want to create and sync the temporary dir. But that doesn't work bc there is no way to check if the dir already exists on the server. syncing the temp dir ("/tmp/a") would override any existing files on the server in the /a dir. So a way to check if a dir exists would also work.

**Additional context**
Add any other context or screenshots about the feature request here.

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