Add primitive types null, true, and false

[Patrick Westerhoff (poke)]

Starting with TypeScript 2, we can use all kinds of literal types. These three are very common literal types which also happen to be reserved words, so I guess it makes sense to handle them in a special way.

Ultimately, it might make sense to expand the syntax structure for literal types a bit but for now, this is a quick fix.

[Microsoft Contribution License Agreements (msftclas)]

Hi Patrick Westerhoff (@poke), I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

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[Mohamed Hegazy (mhegazy)]

Patrick Westerhoff (@poke), sorry for the delay. Can you refresh the PR.

[Sheetal Nandi (sheetalkamat)]

Mohamed Hegazy (@mhegazy) and Patrick Westerhoff (@poke) you shouldn't need this change any more as #257 handles this