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 🚀