[Property accessor - executor] fix bracket accessing on non-objects types

[croraf]

Closes #210
I've added 4 tests, to test accessing properties on functions and strings with both bracket and dot notation.

[codecov]

Codecov Report

Merging #469 into master will increase coverage by 0.05%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #469      +/-   ##
==========================================
+ Coverage   66.39%   66.44%   +0.05%     
==========================================
  Files         146      146              
  Lines        9350     9376      +26     
==========================================
+ Hits         6208     6230      +22     
- Misses       3142     3146       +4     

Impacted Files	Coverage Δ
boa/src/exec/field/mod.rs	73.33% <60.00%> (-8.49%)	⬇️
boa/src/exec/tests.rs	100.00% <100.00%> (ø)
boa/src/builtins/value/conversions.rs	42.42% <0.00%> (-0.44%)	⬇️
boa/src/builtins/value/mod.rs	55.58% <0.00%> (-0.27%)	⬇️
boa/src/builtins/json/tests.rs	100.00% <0.00%> (ø)
boa/src/builtins/json/mod.rs	87.14% <0.00%> (+0.18%)	⬆️

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[Razican]

Looks good to me! Thanks for the contribution :)