Add "viral syntax" to terminology.md

[bnb]

Viral syntax is something that was brought up multiple times in the top-level await discussion. I personally believe I know what it means but it would be nice to have an explicit definition 👍

[gesa]

thrilled you opened this issue but also don't be afraid to resolve it yourself 😉

[littledan]

Note, this term was part of an argument being made my certain TC39 delegates, and we don't have a generally agreed on design principle here. I also haven't heard it used outside of this particular discussion. As we document this term, let's keep that in mind and word things appropriately.