support Luau singleton types RFC

[matthargett]

https://github.com/Roblox/luau/blob/master/rfcs/syntax-singleton-types.md

type NetworkError = "parse" | "connection"
type Element = {
  ["$$typeof"]: number
}
type Cacheable<T> = {
  ["@@meta"]: T
}

[matthargett]

Closed by #219