refactor classes, add private, minor fixes

1-js/09-classes/03-class-inheritance/2-clock-class-extended/task.md

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+importance: 5
+
+---
+
+# Extended clock
+
+We've got a `Clock` class. As of now, it prints the time every second.
+
+Create a new class `ExtendedClock` that inherits from `Clock` and adds the parameter `precision` -- the number of `ms` between "ticks". Should be `1000` (1 second) by default.
+
+- Your code should be in the file `extended-clock.js`
+- Don't modify the original `clock.js`. Extend it.