Consolidate SYCL kernels under a single folder

[antonwolfy]

This PR moves all SYCL kernel functors from extensions/ to kernels/ directory, improving the project structure.

Moved kernels:

• statistics: histogram, sliding_window1d functors
• indexing: choose functor
• window: bartlett, blackman, hamming, hanning, kaiser functors

The PR also improves the code coverage report, since the SYCL kernels can be easy excluded from the report now.

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• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2816/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev4=py313h509198e_17 ran successfully.
Passed: 1354
Failed: 3
Skipped: 16

[coveralls]

coverage: 81.749% (+0.6%) from 81.17%
when pulling e6a1dec on move-functors-to-kernels-dir
into 7fae3a6 on master.