Add HTMLTemplateDocument

[emilgpa]

Fix microsoft/TypeScript#5199

[mhegazy]

👍

[mhegazy]

@zhengbli can we get this in?

[zhengbli]

@mhegazy We can for now, but I found that many of the similar interfaces (HTMLxxxElement) doesn't actually have a legal constructor, while my script generates constructors for them because at the time I thought it would be consistent with the old version (before the script) and reduce breaking changes. Although I think we probably should remove them all together at some point.

[mhegazy]

if we remove the constructors we break instanceof checks. so we need another solution here.

[zhengbli]

In that case we can take this change now which solves this particular issue, and then figure out how to deal with the constructors systematically later.

[zhengbli]

@DasHaus Thansk for your contribution :)