Implement let declaration list instruction (VM)

[jasonwilliams]

Set your JS to this and then run

let a = 1;

You should get undefined as expected. But it will run with no errors

Leaving the JS the same run cargo run --features vm ../tests/js/test.js or Cargo Run (VM) on VSCode.
You will reach an "unimplemented" error, arising from here:
https://github.com/boa-dev/boa/blob/master/boa/src/vm/compilation.rs#L61

This is because Node::LetDeclList hasn't been implemented in this block https://github.com/boa-dev/boa/blob/master/boa/src/vm/compilation.rs#L42-L62

Adding the instruction

You can look at the spiderMonkey Op codes for inspiration, namely DefVar in https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/Bytecode

Then you can add DefVar here:
https://github.com/boa-dev/boa/blob/master/boa/src/vm/instructions.rs#L2-L60

Implementing the instruction

Then here you can push the variable to the right environment, similar to how we do this here:
https://github.com/boa-dev/boa/blob/master/boa/src/syntax/ast/node/declaration/let_decl_list/mod.rs#L44-L48