Feature Request: Settings Option - Delete Branch after Merge

[GeorgeWL]

Proposed Reasoning

My workplace has the policy of "delete branch after Peer Review and Merge", I can imagine others may have the same.
What I'm proposing is a settings option for Delete Branches after Merge

<label>Delete Branches after Merge</label>
<select>
    <option>Always Ask</option>
    <option>Do Not Automatically Delete(default)</option> 
    <option>Automatically Delete Local After Merge</option> 
    <option>Automatically Delete Local and Remote After Merge</option>
</select>

[alexr00]

To verify:

• Install the latest pre-release version of GitHub Pull Requests
• Open a folder which contains a GitHub repo
• Set "githubPullRequests.deleteBranchAfterMerge": true
• Open a PR description from the Pull Requests view
• Merge the PR from the PR description webview
• Verify that the PR branch is deleted