Implement Array.prototype.reduceRight

[benjaminflin]

I implemented Array.prototype.reduceRight pretty similarly to reduce, with a few small changes mostly related to usize constraints.

[codecov]

Codecov Report

Merging #579 into master will increase coverage by 0.09%.
The diff coverage is 85.13%.

@@            Coverage Diff             @@
##           master     #579      +/-   ##
==========================================
+ Coverage   70.40%   70.50%   +0.09%     
==========================================
  Files         175      175              
  Lines       11162    11236      +74     
==========================================
+ Hits         7859     7922      +63     
- Misses       3303     3314      +11     

Impacted Files	Coverage Δ
boa/src/builtins/array/mod.rs	80.66% <78.00%> (-0.31%)	⬇️
boa/src/builtins/array/tests.rs	100.00% <100.00%> (ø)

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