Move label triage to main repo

[mcornella]

Keeping two copies of the same labels is a pain: we have to change the labels.json file when making a label modification in the main repo. We could move the labels.json file to the main repo and also keep track of the appropriate regexp to apply the label. This could be of the following format:

.github/labels.json:

{
	"Area: plugin": "^plugins/\\S+/",
	"Area: theme": "^themes/\\S+",
	...
}

Then, the triage action would grab this file and try to match the regexps, and if it matched, apply the label based on the key.