Add gh prompt to set up gh pages

### Problem

In order to set up a gh pages for your repo, you have to do it through the website afaik.

### Solution

Within the `gh [flags]` format, I'd suggest `pages` either being a new command, or a subcommand of `gh repo`.

It should prompt you to choose a branch and directory like the web interface. Defaults: main & root.

It can also allow you to set custom domains and, less importantly, Jekyll themes & whether to enforce https. At this point, I just listed all the options under /settings/pages for reference.

#### How will it benefit CLI and its users?

It'll make it possible to set up a gh pages for your repo via the CLI.