Display term usage count in the CP Taxonomy term list

[luizboaretto]

As with Statamic V2... it's very important to know how many times a tag or category is being used.

[jacksleight]

This would be great, and hopefully it might be pretty easy to add with https://github.com/statamic/cms/pull/6179.

In the meantime I needed to implement this for a client and came up with this workaround. It's a bit of a hack (and there might be a better way) but it works.