migrate Python build backend to scikit-build-core

[njzjz]

This PR migrates the Python build backend from scikit-build to scikit-build-core, which is being actively developed.

There are no breaking changes to users; all tests should be passed without modification.

The behavior for editable installation (pip install -e .) is changed. Before, the compiled binary libraries (e.g. libdeepmd_op.sp) are installed into the source directory. In this PR, the library files are installed into site-packages separately, while the Python files are still in the source code and can be modified for development.

[codecov]

Codecov Report

Patch coverage: 93.75% and no project coverage change.

Comparison is base (9712c13) 79.37% compared to head (6c41026) 79.37%.
Report is 3 commits behind head on devel.

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #2798   +/-   ##
=======================================
  Coverage   79.37%   79.37%           
=======================================
  Files         244      244           
  Lines       25820    25875   +55     
  Branches     1519     1533   +14     
=======================================
+ Hits        20494    20538   +44     
- Misses       4933     4938    +5     
- Partials      393      399    +6     

Files Changed	Coverage Δ
deepmd/lmp.py	0.00% <0.00%> (ø)
deepmd/op/_add_flt_nvnmd_grad.py	57.14% <ø> (ø)
deepmd/op/_copy_flt_nvnmd_grad.py	100.00% <ø> (ø)
deepmd/op/_dotmul_flt_nvnmd_grad.py	26.66% <ø> (ø)
deepmd/op/_flt_nvnmd_grad.py	100.00% <ø> (ø)
deepmd/op/_gelu.py	68.42% <ø> (ø)
deepmd/op/_map_flt_nvnmd_grad.py	100.00% <ø> (ø)
deepmd/op/_matmul_fitnet_nvnmd_grad.py	100.00% <ø> (ø)
deepmd/op/_matmul_flt2fix_nvnmd.py	90.00% <ø> (ø)
deepmd/op/_matmul_flt_nvnmd_grad.py	85.71% <ø> (ø)
... and 15 more

... and 12 files with indirect coverage changes

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