Add visibility modifier to allow access only within a module

[asfernandes]

Sometimes you want a property/method to be visible outside of a class in the same module but do not want it to be on the public interface outside their module.

For example:

export class PublicClass {
    public myPublicProperty = 1;
    internal myInternalProperty: number;  // do not expose outside this module

    constructor() {
        setValue(this);
    }
}

function setValue(o: PublicClass) {
    o.myInternalProperty = o.myPublicProperty;  // allow access to myInternalProperty outside PublicClass
}

[ghost]

Duplicate of #5228 and #7692.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.