Add rule to detect non-public property access via element access

[ajafff]

class Foo {
    private bar = 1;
}
let foo = new Foo();

foo.bar; // type error
foo['bar']; // can access private property

Although prefer-dot-notation disallows element access where possible, but there are still property names like 'foo-bar' that need to use element access. TypeScript intentionally doesn't check accessibility on element access for reasons ... (see microsoft/TypeScript#26328)

This is likely an error and should be detected by a lint rule

[ajafff]

Depending on how microsoft/TypeScript#26356 and microsoft/TypeScript#26355 are resolved, this rule should detect these cases, too.

[ajafff]

It turns out there are a lot more checks that are simply ignored for element access:

• super['abstractMethod']
• super['prop']
• this['abstractProp'] in constructor
• a['protectedProp']: accessing protected property declared in base class on an instance of a base class in a static method of a derived class (or a function with a this parameter of a derived class)