Add clang-format configuration

[MinyazevR]

This PR aims to add ClangFormat to PythonQt. During the discussion, it was decided that clang-format is a valid solution.
A version of clang-format-20 or higher is required, and a testing script (run-clang-format.sh) has been written for testing and/or running in CI.

It is worth noting that the PythonQt Generator has a style that differs from the main src directory. If this is a priority, you can put a separate .clang-format in the generator directory, which will override the parent one.

The following steps:

1. Run run-clang-format.sh locally
2. If you are satisfied with the formatting result, merge this PR (possibly changing some .clang-format parameters)
3. Merge formatted code
4. Add CI

[mrbean-bremen]

It is worth noting that the PythonQt Generator has a style that differs from the main src directory

This has been discussed before, @usiems wrote:

The generator is composed of sources from different sources, with vastly different styles,

As far as I understand, there is a consensus that a common style would be ok.

[usiems]

The code in generator/simplecpp should be excluded from the reformatting, as this is code from an external source which might be updated again in the future.

[mrbean-bremen]

I had a look at the formatted code, and it mostly looks good to me (with a couple of things that I probably would like to change in the config).
Before continuing with this PR, I would like to wait for the other large PR from the Slicer folks (which may take a while) to avoid a lot of conflicts.

[jcfr]

for the other large PR from the Slicer folks

🙏 I am now working on this and you should expect a series of targeted pull requests 🚀

[mrbean-bremen]

@MinyazevR - I see that you are closing all your PRs - are you giving up on this, or is there a technical reason?
I still would like to have the clang-format config - I'm just not sure about state of the backmerge done by @jcfr.

[jcfr]

@mrbean-bremen In term of backport, I have done everything i wanted to. Thanks for checking in. The remaining parts will be relates to CMake support and can definitely be done afterwards.

Regarding clang, I suggest you exclude the generated_cpp folder.

[MinyazevR]

@mrbean-bremen
Please forgive me. I deleted the repository, forgetting that I had active pull requests in it. I reopened this PR: #326