Extend `copytrito!` for a sparse source

[jishnub]

julia> S = sprand(100, 100, 0.01);

julia> M = zeros(size(S));

julia> @btime copytrito!($M, $S, 'U');
  20.779 μs (0 allocations: 0 bytes) # main
  2.050 μs (0 allocations: 0 bytes) # this PR

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.10%. Comparing base (33fbc75) to head (16076cd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #533 +/- ## ========================================== + Coverage 76.42% 84.10% +7.67% ========================================== Files 12 12 Lines 8969 9020 +51 ========================================== + Hits 6855 7586 +731 + Misses 2114 1434 -680 ```

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[ViralBShah]

Looks straightforward. Please merge when ready.