added Factorial function

[Friendly-Banana]

Provides one part of #26
The new Factorial function uses the symbol ! and provides basic integer factorials. Non-integers are first rounded and then used as integers.

[fkleon]

Looks good pending one comment, thanks for this addition.

Could you also add some unit tests for the parser (into parser_test_set.dart) and the new expression (into expression_test_set.dart)?

[Friendly-Banana]

Ok, I think I added all tests.

[fkleon]

Thanks for adding those tests, this looks good. They've surfaced a few more issues to fix but you're almost there :)

[fkleon]

@Just-Learned-It Thanks for your contribution! Travis CI wasn't working with the new dart format command so I've switched to GitHub Actions and fixed the minor formatting issues on master directly.

[Friendly-Banana]

Glad I could help. I'm using the librabry myself so it's somewhat selfish :). Thanks for all the corrections.
Regarding the format, I had thought about opening another issue for all the format warnings. But you fixing it directly is even better.

[fkleon]

This is included in version 2.3.0 that I've just released to pub