[compiler] Correctly insert (Arrow)FunctionExpressions

Previously we would insert new (Arrow)FunctionExpressions as a sibling
of the original function. However this would break in the outlining case
as it would cause the original function expression's parent to become a
SequenceExpression, breaking a bunch of assumptions in the babel plugin.

To get around this, we synthesize a new VariableDeclaration to contain
the newly inserted function expression and therefore insert it as a true
sibling to the original function.

Yeah, it's kinda gross

ghstack-source-id: df13e3b
Pull Request resolved: facebook#30446

Author: felixshiftellecon