Add classes

Nobody has claimed this yet.

```js
package test

pub class Person {
// private
Name string;
Age i32;

// public
pub ctor(name string, age i32) {
Name = name;
Age = age;
}

pub Get() string {
return Name + " " + Age;
}
}
```

```js
using "test"

package main

var p = test::Person("foo", 42);
print(p.Get());
```

something like that, TBD.

The issue proposes class, constructor, visibility, field, method, package, import, and object-instantiation syntax, but names no files, tests, or entry points. First identify the parser and interpreter entry points that handle declarations and calls, then define the intended syntax and behavior before implementation; done should include working examples like the ones shown.

Written by the indexing model from the issue text.