fix: prevent duplicate buffer name error with setNames

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📃 Summary

With setNames=true, toggling the plugin at least twice result in the following error:

FAIL in tests/test_buffers.lua | left/right | setNames doesn't throw when re-creating side buffers: Vim:Error executing Lua cal
lback: ...lm/Documents/no-neck-pain.nvim/lua/no-neck-pain/wins.lua:76: Vim:E95: Buffer with this name already exists
  stack traceback:
        [C]: in function 'nvim_buf_set_name'
        ...lm/Documents/no-neck-pain.nvim/lua/no-neck-pain/wins.lua:76: in function 'createSideBuffers'
        ...lm/Documents/no-neck-pain.nvim/lua/no-neck-pain/main.lua:44: in function 'init'
        ...lm/Documents/no-neck-pain.nvim/lua/no-neck-pain/main.lua:77: in function 'toggle'
        ...lm/Documents/no-neck-pain.nvim/lua/no-neck-pain/init.lua:11: in function 'toggle'
        .../klm/Documents/no-neck-pain.nvim/plugin/no-neck-pain.lua:18: in function <.../klm/Documents/no-neck-pain.nvim/plugin
/no-neck-pain.lua:17>
  Traceback:
    tests/test_buffers.lua:242

make: *** [test] Error 1

This PR solves this issue by deleting the buffer if it exist.