[Proposal] Add functionality for creating Draft PRs

[bvlion]

Hello @motemen, @onk, and the git-pr-release maintainers,

Currently, git-pr-release lacks the ability to create Draft PRs. This imposes the assumption that all PRs are fully prepared, which may not always reflect the actual situation.

Proposed Solution

I propose to add a setting named pr-release.draft, which should be specified as a boolean value. By default, it will be set to false. However, when users set it to true, the PRs created by git-pr-release will be marked as drafts.

Additional Information

Adding this functionality to git-pr-release would allow users to communicate more clearly to other developers that their work is still ongoing. I believe this could greatly improve the workflow and collaboration within development teams.