Support literal union types

[claabs]

Adds support for unioned literals.

 color: 'red' | 'blue' | 'yellow';

Will output one of the possible values randomly. In fixed mode, it will choose the first literal.

This is written to only support union types that are exclusively literals. Mixed types are not supported:

// Will NOT work
color: 'red' | 'yellow' | string;

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[claabs]

@googlebot I signed it!

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[ryanmcdermott]

This looks great, thank you so much!

[ryanmcdermott]

Because of COVID-19, I'm not able to upload new releases to NPM because I'm not in the office. I'll do this as soon as I'm back there.