Adds 2q prep for iswap instead of sqrt_iswap

[Dripto]

this PR adds a version of the 'prep_two_qubit_state' methods but for a full iswap instead of a sqrt_iswap

[tanujkhattar]

Please add tests and update PR description.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (a98fc61) 97.89% compared to head (8256f11) 97.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6314   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files        1106     1106           
  Lines       96157    96180   +23     
=======================================
+ Hits        94135    94158   +23     
  Misses       2022     2022           

Files	Coverage Δ
cirq-core/cirq/__init__.py	100.00% <ø> (ø)
cirq-core/cirq/transformers/__init__.py	100.00% <ø> (ø)
...transformers/analytical_decompositions/__init__.py	100.00% <ø> (ø)
...ical_decompositions/two_qubit_state_preparation.py	97.95% <100.00%> (+0.59%)	⬆️
...decompositions/two_qubit_state_preparation_test.py	100.00% <100.00%> (ø)

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