Add IMSC profile designators (#1034).

[skynavga]

Closes #1034.

[nigelmegitt]

This is completely contrary to the discussed proposal at #1034 (comment) regarding which @skynavga you seemed to agree, and for which there seemed to be consensus when we discussed it.

[nigelmegitt]

Thanks for addressing #1123 (review) @skynavga , this looks good to me now.

[nigelmegitt]

I see that the normative reference to the profile registry is dated, which matches our normal policy for normative references, but it does mean that, formally, any new profile added to a future revision of the profile registry is excluded from those permitted, by the text:

All profile designators which have the TT Profile Namespace as a prefix but are otherwise not listed in Table 5-2 – Profiles or in [TTML Media Type], §4, Registry, are reserved for future standardization, and must not appear in a conformant document instance.

This seems contrary to the intention of having a registry. @skynavga can we change the reference link to https://www.w3.org/TR/ttml-profile-registry/ for this reference, exceptionally? Alternatively can we modify the text to say

... or in [TTML Media Type], §4, Registry, or in a future revision of [TTML Media Type], are reserved...

?

[skynavga]

@nigelmegitt ok, I made it generic (exceptionally), and also changed the label used in the bibliography to better match the short name of the document; we should probably discuss this exception in a TTWG call to get approval from the WG;

[skynavga]

Tests at w3c/ttml2-tests#250.