Fix: String.prototype.replace substitutions, closes #610

[RageKnify]

This Pull Request fixes/closes #610 .

It changes the following:

• Change the substitution logic in String.prototype.replace so that it more closely follows: https://tc39.es/ecma262/#sec-getsubstitution

[codecov]

Codecov Report

Merging #629 into master will increase coverage by 0.64%.
The diff coverage is 80.76%.

@@            Coverage Diff             @@
##           master     #629      +/-   ##
==========================================
+ Coverage   72.50%   73.14%   +0.64%     
==========================================
  Files         179      192      +13     
  Lines       13376    14005     +629     
==========================================
+ Hits         9698    10244     +546     
- Misses       3678     3761      +83     

Impacted Files	Coverage Δ
boa/src/builtins/string/mod.rs	50.95% <72.22%> (+5.13%)	⬆️
boa/src/builtins/string/tests.rs	100.00% <100.00%> (ø)
...x/parser/expression/primary/function_expression.rs	64.70% <0.00%> (-25.30%)	⬇️
boa/src/syntax/parser/expression/mod.rs	78.48% <0.00%> (-10.92%)	⬇️
boa/src/builtins/object/mod.rs	30.35% <0.00%> (-8.82%)	⬇️
boa/src/exec/switch/mod.rs	59.45% <0.00%> (-8.55%)	⬇️
boa/src/exec/new/mod.rs	66.66% <0.00%> (-8.34%)	⬇️
boa/src/syntax/parser/expression/assignment/mod.rs	62.06% <0.00%> (-7.94%)	⬇️
boa/src/lib.rs	73.91% <0.00%> (-7.91%)	⬇️
boa/src/exec/block/mod.rs	61.11% <0.00%> (-7.64%)	⬇️
... and 89 more

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[RageKnify]

I'm not gonna be working for a week, but I found what I think was an old implementation of the method from V8 in js.
The main change is that we need to go through the replacement string left to right copying characters to the result string and for $ we check if it is part of one of the patterns described in the table (there's a link to the spec in a nearby comment), and if so replace it accordingly.

From a quick glance I think we need to invoke chars on the result string to iterate over them and then peek ahead when we spot $?

[RageKnify]

I've found another bug in the implementation of `String.prototype.replace', copied from Chrome:

> let str = 'boa';
> str.replace(/.*/, "??$&??")
"??boa??"
> str.replace(".*", "??$&??")
"boa"

While on boa both would return "??boa??" because the string ".*" is converted to a RegEx pattern, I'm gonna try to make String.prototype.replace follow the spec more closely.

[RageKnify]

I ended up not fixing the other bug, my intention was to follow the spec, but it requires RegExp.prototype [ @@replace ] to be implemented for invocations of String.prototype.replace to work and right now I don't believe it is defined/implemented I think we have an approximation of the implementation here in String.prototype.replace.

I think symbol_properties are working fine, we're just missing the implementation of the function and the initialization of it at startup.

Another thing to keep in mind is regress, which was mentioned in #478 I believe regress's API is lacking right now but it would allow boa not to worry about implementing RegEx. Currently regress isn't very active, but we could try to get it going, probably try to identify which methods we would need and go from there.

[RageKnify]

When I get back I'll implement tests for $$, $&, $', $nn, '$`'

PS: Just now realized that I could use tuple pattern matching instead of using matchs inside matchs. I'll try to use that, it'll make it more understandable and shorter.

[Razican]

It looks pretty good :) check my comments, they are mostly for my understanding. Also, in the future, we will want to move to regress for regular expressions, as in #478, and it might in fact prove to be easier to implement with it.

[Razican]

This is good to know from my side now!

[RageKnify]

@jasonwilliams @HalidOdat @Lan2u sorry to tag, but it's been waiting for a 2nd review for a week

[HalidOdat]

This looks good! check my comments to see on how we might improve it :)

Sorry for the long delay I haven't had that much free time this week.