Missing spec for `org.matrix.preview_urls` account data

[Half-Shot]

Element Web (and Classic mobile apps) use org.matrix.preview_urls as a cross account setting to enable/disable showing a preview of a URL under a message. I think this has been around since at least 2017 based on issues, but I think @ara4n may have implemented it even earlier.

I also suspect some clients in the ecosystem also make use of it, which is why I've put it in this repository rather than a bug against Element.

[richvdh]

Obviously, this would need an MSC, and the account data item would need a better name that doesn't have the org.matrix prefix.