build: Set "root": true, in .eslintrc.json

This ensures that each repository's "npm test" command is fully
independent of wherever it might be in the filesystem.

Bug: T206485
Change-Id: I5ec93fff138f9821e151b28f6ecd9f891e4912c6

.eslintrc.json

@@ -1,4 +1,5 @@
 {
+	"root": true,
 	"extends": [
 		"wikimedia/client",
 		"wikimedia/jquery"