feat(js-checkpoint):add freeze exercise

subjects/freeze/README.md

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+## freeze
+
+### Instructions
+
+Create the following constant variables:
+
+- `arr` : an array containing the values `4` and `2`.
+- `obj`: an object containing primitive values:
+  - `str`: with a `string` value.
+  - `num`: with a `number` value.
+  - `bool`: with a `boolean` value.
+  - `undef`: with a `undefined` value.
+
+`arr` and `obj` must be frozen, so that their elements or properties cannot be changed.