Restructure dpctl, delegating all array functionality to dpnp

[antonwolfy]

Tasks:

• Add new header file dpnp4pybind11.hpp #2734
• Move usm_ndarray API class into dpctl/tensor #2749
• Move _tensor_impl extensions and use it for dpnp #2755
• Extend ._tensor_impl with remaining functions used by dpnp #2758
• Extend ._tensor_impl with copy functions  #2774
• Extend ._tensor_impl with advanced indexing functions #2777
• Extend ._tensor_impl with where(), clip() and type utils functions #2778
• Extend ._tensor_impl with linear sequences functions  #2782
• Move _tensor_accumulation_impl extension and use it for dpnp #2791
• Move _tensor_sorting_impl extension and use it for dpnp #2793
• Move _tensor_reductions_impl extension and use it for dpnp #2794
• Move _tensor_elementwise_impl (unary) extension and use it for dpnp #2795
• Extend _tensor_elementwise_impl (unary) part 2 #2796
• Extend _tensor_elementwise_impl (unary) part 3 #2801
• add tensor linalg extension #2799
• Extend _tensor_elementwise_impl with binary functions and use it for dpnp  #2803
• Extend _tensor_elementwise_impl with binary functions part 2 #2804
• Extend _tensor_elementwise_impl with binary functions part 3 #2805
• Extend dpctl_ext.tensor with the remaining functions  #2806
• Move usm_ndarray into dpctl_ext.tensor #2807
• Move dpctl_ext.tensor to dpnp.tensor #2829
• Remove dpctl_ext.tensor C-API  #2830
• Add __main__.py for CLI options to get libtensor include dirs from module #2836
• Move compute follows data utils #2840
• Add device-aware output dtype for dpt.round() with boolean input #2851
• Migrate dpctl.tensor into dpnp.tensor #2856
• Update CHANGELOG.md with dpnp.tensor migration #2857
• Use dpctl_capi.h header in dpnp4pybind11.hpp #2859
• Include dpctl4pybind11.hpp in dpnp4pybind11.hpp and switch libtensor to dpnp namespace #2862

Tests:

• Move tensor tests from dpctl  #2817
• Enable conditional execution of Array API tests for dpctl_ext in GH Action #2823
• Move dpctl_ext.tensor to dpnp.tensor #2829
• Fix test warnings in dpnp.tensor tests #2852

Docs:

• Add cross-reference fallback for dpnp.tensor to dpctl 0.21.1 docs #2848

[ndgrigorian]

@vlad-perevezentsev @antonwolfy
As final step, I believe some of tensor-related utility functions from dpctl in __main__.py should be added:
https://github.com/IntelPython/dpctl/blob/master/dpctl/__main__.py
these are focused on getting paths for builds, extension writing, etc.

[antonwolfy]

The remaining work will be done in the follow-up PRs. Closing the task as done.