revamp GoToTargetUndirected & GoToTargetDirected

[Sid-Bhatia-0]

1. Remove old GoToTargetUndirected and GoToTargetDirected.
2. Add new GoToTargetUndirectedModule.GoToTargetUndirected and GoToTargetDirectedModule.GoToTargetDirected.
3. Update tests.
4. Update README.

Here is how the new environments look:

GoToTargetUndirectedModule.GoToTargetUndirected:

GoToTargetDirectedModule.GoToTargetDirected:

[codecov-commenter]

Codecov Report

Merging #159 (609315e) into master (9a2f104) will decrease coverage by 0.69%.
The diff coverage is 79.08%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   71.03%   70.33%   -0.70%     
==========================================
  Files          27       28       +1     
  Lines        2479     2488       +9     
==========================================
- Hits         1761     1750      -11     
- Misses        718      738      +20     

Impacted Files	Coverage Δ
src/envs/envs.jl	72.72% <ø> (ø)
src/envs/go_to_target_directed.jl	72.88% <72.88%> (ø)
src/envs/go_to_target_undirected.jl	82.05% <82.05%> (ø)
src/rlbase.jl	84.52% <87.50%> (+0.70%)	⬆️

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