[WIP] Add `Shadow` transform

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 87.87%. Comparing base (c5bf155) to head (4e04bda).

Files	Patch %	Lines
src/transforms/shadow.jl	0.00%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #878 +/- ## ========================================== - Coverage 88.07% 87.87% -0.21% ========================================== Files 172 173 +1 Lines 5175 5187 +12 ========================================== Hits 4558 4558 - Misses 617 629 +12 ```

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