[llvm] [Py Reformat] Exclude `third-party` in `code-format-helper.py` (PR #83872)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

@tru I'm not sure the current setup is very maintainable - my concern is scenarios where one uses (say) vscode for development and wants to auto-format. Ideally they'd say "using `darker`, with whatever project defaults" and move on. Right now that's not quite the case, since (iiuc) the project settings are ignored by darker - or perhaps only the file exclusion list is (or maybe I'm not using the tools well)

https://github.com/llvm/llvm-project/pull/83872