Implement generators

[Razican]

ECMASCript feature
Currently, Boa has no support for Generators. Generators can be synchronous or asynchronous, and you can find the specs here:

• Generator Function Definitions: https://tc39.es/ecma262/#sec-generator-function-definitions
• Async Generator Function Definitions: https://tc39.es/ecma262/#sec-async-generator-function-definitions

This first needs changes in the parser, to check if a the function being defined is a generator or not. Will almost certainly require new AST nodes for that.

Then, it will require implementing the "Runtime Semantics" on the execution side. This will require implementing the new methods mentioned in the spec, and maybe some changes on how we handle #367. This might also collide with #818, since async generators will certainly require asynchronous execution that will be implemented there.