Discussed with @DHowett-MSFT, as a follow up of #2554. See also #2915.

In #2554 I'm not adding support for parsing these additional extended states, because their implementation seems closely tied to more complicated existing implementations.

• Faint cannot be turned on at the same time as Bold
• Underlined is a trinary state
• We're currently using LVB_UNDERSCORE to store "underlined"

Due to the timing with the upcoming 20H1 release, I felt it too risky to try and implement those as well. I've left room in the ExtendedAttributes enum for these states, but currently, Doubly-Underlined and Faint are totally ignored.

We'll need to parse and store them in the buffer correctly. It's another task entirely to support rendering these things correctly. I believe the rendering for underline state will be satisfied with #2915. Faint might just be okay too, considering it's like the opposite of Bold.