Rework implementation of `dpnp.fabs` function

[antonwolfy]

The PR proposes to rework implementation of dpnp.fabs function which is quite limited now.

It assumes to have a separate _ufunc_impl pybind11 extension to aggravate all universal functions which are not provided by dpctl tensor implementation, but requires to be aligned with NumPy.

The new extension is implemented following similar approach as for _vm_impl extension and proposes to reuse existing UnaryElementwiseFunc functionality from dpctl tensor.

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• [x] Have you tested your changes locally for CPU and GPU devices?
• [x] Have you made sure that new changes do not introduce compiler warnings?
• [ ] Have you checked performance impact of proposed changes?
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