Add configuration settings for git worktree folder

[kvokka]

I'm running Sidecar in devcontainer, where the working dir is /workspace. To keep worktrees persistent across container rebuild it's required to keep them inside working directory, but Sidecar hardcoded that all worktrees should be on the same nested level with the project.

It results in the situation, that Sidecar is trying to create new worktree in the / and fail with fails with Permission denied error.

As the workaround the path for the worktrees should be configurable, allowing to setups like mine to work.

❯ sidecar --version
sidecar version 0.71.1

I'm ok to fix this if it's ok