[WIP] Add options to checkout PR destination branch

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Checkout PR target branch after merge/branch delete</issue_title>
<issue_description>- Extension version: Remote - SSH v0.96.0

• VSCode Version: 1.76
• OS: Win

After merging a PR and deleting branches the default branch is checked out.
It'd be more inline with the user flow to check out the PR destination branch instead (
optionally with a message to pull the just merged commits)</issue_description>

<agent_instructions>Add another two options to the postDone setting: checkoutPullRequestBaseBranch and checkoutPullRequestBaseBranchAndPull. Implement these options.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Checkout PR target branch after merge/branch delete #4582

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