feat(workspaces): add +workspace/destroy

[sriramsk1999]

Adds functionality to permanently remove saved workspaces. There's already a +workspace/kill and +workspace/delete which provide different functionality, so I've added +workspace/destroy instead to remove workspaces on disk.

This is my first time writing any nontrivial elisp, so I'm sure there's plenty of issues, will be happy to address them :)

Ref: #4399
Close: #4399

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• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to the git conventions.
• Any relevant issues or PRs have been linked to.