Implement coalescing (?? and ??=)

[tofpie]

This Pull Request fixes/closes #1012.

It changes the following:

• Lexing of ?? and ??=
• Parsing of logical expressions
• Execution of ?? and ??=

[codecov]

Codecov Report

Merging #1013 (d3d4acb) into master (6f3641d) will increase coverage by 0.01%.
The diff coverage is 56.98%.

@@            Coverage Diff             @@
##           master    #1013      +/-   ##
==========================================
+ Coverage   59.83%   59.85%   +0.01%     
==========================================
  Files         167      167              
  Lines       11018    11088      +70     
==========================================
+ Hits         6593     6637      +44     
- Misses       4425     4451      +26     

Impacted Files	Coverage Δ
boa/src/syntax/ast/op.rs	10.98% <0.00%> (-0.25%)	⬇️
boa/src/syntax/ast/punctuator.rs	6.00% <0.00%> (-0.25%)	⬇️
boa/src/syntax/lexer/mod.rs	66.66% <0.00%> (ø)
boa/src/syntax/ast/node/operator/bin_op/mod.rs	68.50% <41.66%> (-1.50%)	⬇️
boa/src/syntax/parser/expression/mod.rs	81.89% <68.00%> (-10.65%)	⬇️
boa/src/syntax/lexer/operator.rs	70.17% <72.72%> (+0.61%)	⬆️
...syntax/parser/expression/assignment/conditional.rs	85.00% <100.00%> (ø)
...ntax/parser/expression/left_hand_side/arguments.rs	63.15% <0.00%> (+5.26%)	⬆️

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[Razican]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,493	78,493	0
Passed	19,787	19,824	+37
Ignored	15,585	15,585	0
Failed	43,121	43,084	-37
Panics	396	396	0
Conformance	25.21	25.26	+0.05%