refactor: move ChainRulesCore into an extension

[avik-pal]

ChainRulesCore isn't needed by the core framework, but still adds some load time. Anyone requiring the functionality will have loaded it by default...

julia> @time using WeightInitializers
  0.040279 seconds (43.41 k allocations: 3.302 MiB)  # NOW

julia> @time using WeightInitializers
  0.060959 seconds (63.61 k allocations: 4.682 MiB)  # BEFORE

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.80%. Comparing base (0d95f4a) to head (129a516).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #36   +/-   ##
=======================================
  Coverage   93.80%   93.80%           
=======================================
  Files           9        9           
  Lines         210      210           
=======================================
  Hits          197      197           
  Misses         13       13           

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