Fix incomplete support for exponential operator

[danez]

While looking through the es2016+ table here I noticed that babel is not marked green for exponentiation (**) operator - basic support. This has to do with the parenthesis that are mandatory for prefixed literals.

So ultimately it should be like this:

5 ** 6; // success
(-5) ** 6; // success
-(5 ** 6); // success
-5 ** 6; // fail
-(5) ** 6; // fail
(-5 ** 6); // fail

But currently it is like this:

5 ** 6; // success
(-5) ** 6; // fail
-(5 ** 6); // success
-5 ** 6; // fail
-(5) ** 6; // success
(-5 ** 6); // fail

I added some currently failng tests.

[danez]

I found a solution to fix it. Luckily we already have a lot of information about parenthesis in extra.

[hzoo]

👍

[codecov-io]

Current coverage is 97.00%

Merging #75 into master will not change coverage

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