add Snake environment

[Sid-Bhatia-0]

1. Add DataStructures package
2. Add Snake environment Add SnakeGame #122
3. Add tests and gif for Snake

[codecov-io]

Codecov Report

Merging #123 (ca54bb1) into master (dedfa11) will increase coverage by 0.23%.
The diff coverage is 81.48%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
+ Coverage   76.65%   76.88%   +0.23%     
==========================================
  Files          17       18       +1     
  Lines         848      930      +82     
==========================================
+ Hits          650      715      +65     
- Misses        198      215      +17     

Impacted Files	Coverage Δ
src/GridWorlds.jl	100.00% <ø> (ø)
src/objects.jl	34.04% <0.00%> (-1.68%)	⬇️
src/envs/snake.jl	85.71% <85.71%> (ø)
src/envs/dynamic_obstacles.jl	97.56% <0.00%> (-1.22%)	⬇️
src/envs/sequential_rooms.jl	99.11% <0.00%> (-0.89%)	⬇️

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