Add support for actions

[Sid-Bhatia-0]

1. Creating an abstract type AbstractGridWorldAction for actions.
2. Adding a default implementation for get_actions that can be extended by specific environments.
3. Fixed variable name from dir to action since TURN_LEFT and TURN_RIGHT are actions.

[codecov-io]

Codecov Report

Merging #40 into master will decrease coverage by 0.12%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   37.67%   37.54%   -0.13%     
==========================================
  Files          13       13              
  Lines         292      293       +1     
==========================================
  Hits          110      110              
- Misses        182      183       +1     

Impacted Files	Coverage Δ
src/actions.jl	100.00% <ø> (ø)
src/abstract_grid_world.jl	77.77% <50.00%> (-4.58%)	:arrow_down:

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[findmyway]

Well done!