Make IMSC-HRM an "updateable Rec"

[nigelmegitt]

I thought TTWG had decided to adopt the "updateable Recommendation" approach defined at https://www.w3.org/2023/Process-20231103/#allow-new-features but I don't have a proper record of that, nor does the specification that we approved for requesting transition to PR have it.

Some discussion in 2020, not clearly leading to a resolution: https://www.w3.org/2020/11/12-tt-minutes.html#t03

Mechanics for implementing: there's a Respec class that can be added, as documented at https://respec.org/docs/#updateable-rec-class

We effectively have one shot at doing this, which is before Rec, so this is a showstopper that I only noticed when I saw that w3c/transitions#590 says:

Will new features be allowed to be incorporated in the Recommendation?

No.

which surprised me. I'd like to propose that the TTWG considers this and approves making it an updateable Rec.

[nigelmegitt]

I updated #77 directly to make this an updateable Rec, as per the CfC at https://lists.w3.org/Archives/Public/public-tt/2024Jan/0006.html .

[nigelmegitt]

Closing as done in #77.