Use <!doctypehtml> (without the space) where possible

Use <!doctypehtml> (without the space) where possible

When both the `useShortDoctype` and the `removeTagWhitespace` options are enabled, we can remove the space from the doctype altogether.

It’s invalid HTML (which is already expected in the output with the `removeTagWhitespace` option enabled), but it still triggers standards mode.

https://html.spec.whatwg.org/multipage/parsing.html#parse-error-missing-whitespace-before-doctype-name
https://twitter.com/mathias/status/1048098677703753729

Closes #969.