Merge Lightning Qubit and Lightning Kokkos backends in a single improved repository

[AmintorDusko]

Context: This PR creates an improved structure for the Lightning repository, and merges Lightning Qubit and Lightning Kokkos in a single repository.

Description of the Change:

Complete overhaul of repository code.

• New modular build system.
• New device-agnostic and device-specific code separation, with code reuse between backends.
• New partial C++ tests with restricted build.
• New setup build with namespace packaging.

Benefits:
All of them.

Possible Drawbacks:
Not aware of it.

Related GitHub Issues:

[AmintorDusko]

The update dev version action will continue to fail because the script looks for the version in a single place and the _version.py file changed place. Not much to be done here.

[AmintorDusko]

[sc-38802]

[codecov]

Codecov Report

Merging #472 (41fd729) into master (dfd715e) will decrease coverage by 2.70%.
The diff coverage is 94.80%.

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
- Coverage   99.91%   97.22%   -2.70%     
==========================================
  Files          50      142      +92     
  Lines        4671    16420   +11749     
==========================================
+ Hits         4667    15964   +11297     
- Misses          4      456     +452     

Files Changed	Coverage Δ
...asurements/tests/Test_StateVectorKokkos_Expval.cpp	100.00% <ø> (ø)
...surements/tests/Test_StateVectorKokkos_Measure.cpp	100.00% <ø> (ø)
.../measurements/tests/Test_StateVectorKokkos_Var.cpp	100.00% <ø> (ø)
...lightning_kokkos/observables/ObservablesKokkos.hpp	100.00% <ø> (ø)
...okkos/observables/tests/Test_ObservablesKokkos.cpp	100.00% <ø> (ø)
...lightning_kokkos/tests/Test_StateVectorLKokkos.cpp	100.00% <ø> (ø)
...ors/lightning_kokkos/utils/LinearAlgebraKokkos.hpp	100.00% <ø> (ø)
...ightning_kokkos/utils/tests/Test_LinearAlgebra.cpp	100.00% <ø> (ø)
...c/simulators/lightning_qubit/StateVectorLQubit.hpp	100.00% <ø> (ø)
...ators/lightning_qubit/StateVectorLQubitManaged.hpp	100.00% <ø> (ø)
... and 132 more

... and 9 files with indirect coverage changes

[AmintorDusko]

Codecov is activated, and the upload pypi actions are present.
I will open the PR for review now, since it is extensive, while I work in parallel on what still needs to be done.

[AmintorDusko]

I remember that docs and benchmarks will come in follow-up PRs.

[vincentmr]

Few questions and suggestions, but look good to me.

[mlxd]

Thanks for all the hard work here!
Approving, and overriding the dev version failure, which will be rectified upon merging this PR

[vincentmr]

I think this is good to go. Nice work tying all the strings together.