VM: Implement variable declaration (var, const, and let)

[AnnikaCodes]

This Pull Request fixes #1028.

It changes the following:

• Adds a DefVar VM instruction
• Includes Context in the function signature of CodeGen::compile
• Compiles LetDeclLists into DefVar instructions

[codecov]

Codecov Report

Merging #1030 (d8d2c27) into master (1052ccd) will decrease coverage by 0.32%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1030      +/-   ##
==========================================
- Coverage   58.72%   58.39%   -0.33%     
==========================================
  Files         172      172              
  Lines       11728    11861     +133     
==========================================
+ Hits         6887     6926      +39     
- Misses       4841     4935      +94     

Impacted Files	Coverage Δ
boa/src/vm/compilation.rs	0.00% <0.00%> (ø)
boa/src/vm/instructions.rs	0.00% <0.00%> (ø)
boa/src/vm/mod.rs	0.00% <0.00%> (ø)
boa/src/builtins/object/mod.rs	62.29% <0.00%> (-5.57%)	⬇️
boa/src/builtins/regexp/mod.rs	67.25% <0.00%> (-2.88%)	⬇️
boa/src/object/mod.rs	51.36% <0.00%> (-0.15%)	⬇️
boa/src/value/display.rs	85.00% <0.00%> (ø)
boa/src/object/internal_methods.rs	62.93% <0.00%> (ø)
boa/src/object/gcobject.rs	67.93% <0.00%> (+0.34%)	⬆️
boa/src/builtins/string/mod.rs	62.80% <0.00%> (+1.46%)	⬆️

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[AnnikaCodes]

I made this support var and const as well.

[jasonwilliams]

Could we add some documentation/comments to these? So others know what the 2 usize arguments are for
As a bonus putting these in some alphabetical order would be nice (but not essential)

[Razican]

We could even use attributes in a struct-like variant, with names. Would make things much more clear.

[jasonwilliams]

let a = 1;

generates

[boa/src/context.rs:761] &compiler = Compiler {
    instructions: [
        DefLet(
            0,
        ),
        One,
        InitLexical(
            0,
        ),
    ],
    pool: [
        String(
            RcString(
                "a",
            ),
        ),
    ],
}
1

@AnnikaCodes if you look at the changes ive made, letDecl now doesn't use run instead it calls compile and that in terms puts One on the stack. We will need to do something similar with Var and Const.

There's optimization that can be done here, we know DefLet will always be a string, so we don't need to convert it back and forth between a Value and a String. It will be converted from a &str to a Value when the instruction is created, then back to a String when we make the binding. We can defer this for now though and look at it at a later date

[Razican]

From my side, I think a bit of documentation is needed in the definition of the enums. Give it a look, but from the rest, I'm happy with it :)

[jasonwilliams]

Yep looks good to me.
Thanks for your contribution AnnikaCodes !