Extend no-invalid-this rule to disallow this in functions in methods

[inthemill]

This PR extends the [no-invalid-this] with the option "in-function-in-method".
If it is enabled it complains about the usage of the this keyword inside of function expression within a method.
this in such function expressions is usually meant to refer the this outside of the functions scope.
This rule will not complain about this in arrow functions.

In our project, it happened often when migrating JS to TS. With an angular service written as

angular.module('a',[]).factory('Service', function(Helper, Enhancer){
return {
    getAllMapped: function(){
         return Helper.getAll()
             .map(function(item){
                  return Enhancer.enhance(item);
             })
    }
}
}); 

We often end up with

class ServiceClass {
    constructor(private Helper, private Enhancer){
    }

    public getAllMapped() {
         return this.Helper.getAll()
             .map(function(item){
                  return this.Enhancer.enhance(item);
             })
    }
}
angular.module('a',[]).service('Service', ServiceClass);

which will not work as this.Enhancer is not defined.

[jkillian]

Can we keep this behavior on by default? For backwards-compat reasons, we don't want to have to make users add a new option to their rule to get the same behavior as before

[jkillian]

Looks good to me logic-wise @inthemill! If you could just make the couple tweaks listed above about the rule's API and style things, we'll be able to merge this after.

[jkillian]

Looks great, thanks @inthemill!