Import block metadata into plugin directory on commit

[tellyworth]

Currently, block metadata (as per #4345) is imported irregularly by a script. It should happen on commit, as per the other plugin metadata.

[tellyworth]

Add importer code

[dd32]

@tellyworth 4393.diff​ looks good to me, I see nothing that stands out as potentially being a problem.

Seems like it's also future-proof enough in that it'll be reasonably easy to expand upon it later, for example storing more data.

I suspect we'll probably have to find another way to store the data for search purposes though? but storing data like this and breaking it out into whatever fields (or taxonomies) needed later for that seems simple enough.

[dd32]

Looks like the relevant commits missed this ticket [8868], [9052] [9137].