Implement the comma operator

[kashparty]

This Pull Request fixes/closes #569 .

It changes the following:

• Adds the comma operator
• Changes assignment operator parsing so that the comma operator is not parsed as part of the assignment (explained below)
• Adds tests

Examples:

var a, b;
b = 10;
a = (b++, b);

// a is now 11, b is now 11

var a, b;
b = 10;
a = (b += 5, b - 1);

// a is now 14, b is now 15

var a, b;
b = 10;
a = (b += 5, b *= 2, b - 1);

// a is now 29, b is now 30

The reason why assignment operator parsing is changed is because otherwise, for cases like the second example, the parser continues to parse the tokens after the comma operator as part of the assignment. For the second example, this would cause the b to be assigned the value of 5, b - 1 (which is b - 1). So I made a minor change to the assignment operator parsing so that it stops parsing when it reaches a comma operator.

[codecov]

Codecov Report

Merging #589 into master will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #589   +/-   ##
=======================================
  Coverage   71.10%   71.10%           
=======================================
  Files         177      177           
  Lines       11420    11428    +8     
=======================================
+ Hits         8120     8126    +6     
- Misses       3300     3302    +2     

Impacted Files	Coverage Δ
boa/src/syntax/ast/op.rs	9.19% <0.00%> (-0.11%)	⬇️
boa/src/syntax/ast/punctuator.rs	2.10% <0.00%> (-0.03%)	⬇️
boa/src/exec/operator/mod.rs	68.83% <66.66%> (+0.61%)	⬆️
boa/src/exec/tests.rs	100.00% <100.00%> (ø)
boa/src/syntax/parser/expression/assignment/mod.rs	70.00% <100.00%> (ø)

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