Type for hardcoded string values that aren't enum values

[dontcallmedom]

e.g. MediaStreamTrack.kind isn't an enum (because of whatwg/webidl#893), but can only take a few well-defined hardcoded values; it would be good if they could be marked-up in a way that reflects their token-like nature. (including considering #5 and #3 )

[marcoscaceres]

Related #8

[dontcallmedom]

a few other examples of hardcoded strings that could use this for lack of a more detailed taxonomy:

• Do Not Upgrade and Upgrade in Upgrade Insecure Requests (currently marked up with the dfn-type "value" which is supposed to be used only for CSS values)
• WebDriver keywords (see a recent case where they were described as WebIDL enums instead)

(there probably should also be a broader discussion on extending the taxonomy of known definition types, for HTTP header values, for WebDriver keywords - but I think a discussion on a generic fallback still has merits)