Async function expression execution

[Razican]

ECMASCript feature
Boa is already capable of parsing async functions, but it's not able to execute them. The current implementation just returns an "undefined" value if one gets executed:

boa/boa/src/bytecompiler.rs

Lines 879 to 882 in 672eede

	// TODO: implement AsyncFunctionExpr
	Node::AsyncFunctionExpr(_) => {
	self.emit_opcode(Opcode::PushUndefined);
	}

You can learn about runtime semantics here: https://tc39.es/ecma262/#sec-runtime-semantics-instantiateasyncfunctionobject

You will need to implement async function objects:

• https://tc39.es/ecma262/#sec-async-function-objects

Note that the implementation of await is out of the scope of this issue.

[raskad]

Implemented in #2158