Prevent creating a MR on a non-existing branch

Problem to solve

Creating a MR from an existing branch to a non-existing branch proceeds successfully in the CLI, while the web interface displays an error message.

Proposal

Check whether the MR's target branch exists on the project's repository, and prevent users from creating merge requests on non-existing branches.

Further details

This issue is loosely related to #979

➜  glab-mr-to-non-existing-branch git:(main) glab mr create -b develop
? Choose a template Open a blank merge request
? Title: this is a test
? Description <Received>
? What's next? Submit

Creating merge request for main into develop in william-atyos/glab-mr-to-non-existing-branch

!1 this is a test (main)
 https://gitlab.com/william-atyos/glab-mr-to-non-existing-branch/-/merge_requests/1

glab version : glab version 1.34.0 (2023-10-21)

Links / references

See this MR for reference : william-atyos/glab-mr-to-non-existing-branch!1