Support more number literals in BigInt's from string constructor

[georgeroman]

This fixes the cannot convert to BigInt errors documented in #817.

[codecov]

Codecov Report

Merging #880 into master will increase coverage by 0.21%.
The diff coverage is 70.86%.

@@            Coverage Diff             @@
##           master     #880      +/-   ##
==========================================
+ Coverage   59.08%   59.30%   +0.21%     
==========================================
  Files         164      165       +1     
  Lines       10209    10342     +133     
==========================================
+ Hits         6032     6133     +101     
- Misses       4177     4209      +32     

Impacted Files	Coverage Δ
boa/src/builtins/mod.rs	23.52% <ø> (ø)
boa/src/syntax/ast/op.rs	11.23% <ø> (ø)
boa/src/object/mod.rs	46.99% <40.00%> (-0.11%)	⬇️
boa/src/syntax/lexer/number.rs	63.76% <40.00%> (+0.35%)	⬆️
boa/src/builtins/bigint/conversions.rs	76.31% <66.66%> (-5.83%)	⬇️
boa/src/builtins/function/mod.rs	63.47% <66.66%> (+0.37%)	⬆️
boa/src/builtins/map/map_iterator.rs	71.42% <71.42%> (ø)
boa/src/builtins/map/mod.rs	72.26% <80.00%> (+0.95%)	⬆️
boa/src/builtins/array/mod.rs	73.51% <85.18%> (+0.80%)	⬆️
boa/src/builtins/iterable/mod.rs	94.23% <100.00%> (+0.35%)	⬆️
... and 4 more

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[HalidOdat]

Looks good to me! :)

[Razican]

Looks very good! thanks for the contribution :D