Add `public #x`

We don't know what is private #x yet (I hope #4 will be answered soon), but it seems to be some kind of symbol.

It makes sense to have public #x notation in this case, doesn't it?
I propose it to be kinda like shorthand for const x = Symbol(). But in this case, it have much more sense to have class-fields affected like in #1.

Examples:

public #x;
existingObj.#y = 1; // throws because #y isn't declared yet
const obj = {
    #x: 1,
    public #y: 1,
};
const otherObj = {
    public #x: 1, // shadows `#x` from outer lexical scope
    #y: 1, //  throws because #y isn't declared yet in this scope
};

public #x;
existingObj.#y = 1; // throws because #y isn't declared yet
class SomeClass {
    #x = 1;
    public #y = 1;
};
class SomeOtherClass = {
    public #x = 1; // shadowed `#x` not shared with `SomeClass`
    public #y = 1; // another `#y` not shared with `SomeClass`
};

No files or tests are named; begin with the examples in this issue and review issue #4, then compare the interaction with class fields described in #1. Done means the semantics for public private-name declarations and the object and class examples are agreed and documented.