Decorate anonymous function variable assignment

[iddan]

Similar to the rule applied in https://www.ecma-international.org/ecma-262/#sec-isanonymousfunctiondefinition variable assigned to function expressions could be decorated.

This is important for readability especially when multiple decorators are applied and it is an easier solution for declaring decorated functions.

instead of:

const f = @g () => 0

It will be possible to write:

@g
const f = () => 0

And it will be transpiled to:

const f = g(function f() { return 0; });