Handle union of string values

[epabst]

Fixes https://github.com/Kotlin/ts2kt/issues/95

[epabst]

Rebased and updated based on feedback.

[Schahen]

@epabst it looks like comments meta-information is broken in
testData/typeAlias/typeParams.d.kt

Can you please verify on your side as well?

[epabst]

Great catch! What do you think it should look like since nesting the comments won't compile?

[Schahen]

Let's discuss. It's seems to me following options are viable.

• Just leave String | List<String | Number> because actually the only way those comments are helpful now is as hints - and this is a perfectly valid hint.
• Omit String | Number part completely.

First option looks nicer to me)

[epabst]

@Schahen I've fixed the issue with losing the comments and improved them as you've suggested. The fix initially broke nullability, which I then fixed, including adding more tests. The fix had a side-effect of preserving the original order of unioned types. Please re-review.