Implementation of JsMap Wrapper (boa-dev#2115)

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This Pull Request related to JsMap for boa-dev#2098.

Any feedback on implementing JsMapIterator would be welcome. I wasn't entirely sure if it was the right approach, but as I worked on the example file, it felt like something at least similar would be needed to use Map's .entries(), .keys(), and .values() methods.

It changes the following:

- Implements JsMap Wrapper
- Implements JsMapIterator Wrapper
- Creates JsMap example in boa_examples