Add default parameter support

[0x7D2B]

Modern JavaScript has support for default parameters in functions. Boa has support for the syntax but currently default parameters are ignored. For example, this code:

function test(a, b = 2, c = a + b) {
    console.log(a, b, c);
}
test()
test(1)

Result produced by current Boa master:

undefined undefined undefined
1 undefined undefined
undefined

Expected result:

undefined 2 NaN
1 2 3
undefined

This PR implements support for this feature.

[codecov]

Codecov Report

Merging #988 (6c7f95f) into master (880792e) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #988   +/-   ##
=======================================
  Coverage   59.67%   59.67%           
=======================================
  Files         166      166           
  Lines       10813    10823   +10     
=======================================
+ Hits         6453     6459    +6     
- Misses       4360     4364    +4     

Impacted Files	Coverage Δ
boa/src/object/gcobject.rs	63.34% <50.00%> (-0.87%)	⬇️
boa/src/syntax/ast/node/mod.rs	49.60% <0.00%> (+1.57%)	⬆️

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[0x7D2B]

Need to handle scope correctly (function body has a separate scope if default parameters are used: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters#Scope_Effects) but can't really test this due to #989.

[Razican]

If I understand this correctly, this is blocked on #989, right?

[0x7D2B]

Yeah. I have a commit that sets up the right scopes in theory, but it doesn't work correctly due to issues caused by #989 (the second scope effects example given on the MDN page is a good showcase of the bug) so I can't fully finish this and add tests.

[Razican]

Hello @0x7D2B is there any progress on this?

[0x7D2B]

I should probably restart this one from scratch.