Update eslint and c8 to support using TS interfaces

This will allow us to define the repository interface

ghost/collections/.eslintrc.js

@@ -1,7 +1,11 @@
 module.exports = {
     parser: '@typescript-eslint/parser',
-    plugins: ['ghost'],
+    plugins: ['ghost', '@typescript-eslint'],
     extends: [
         'plugin:ghost/node'
-    ]
+    ],
+    rules: {
+        'no-unused-vars': 'off',
+        '@typescript-eslint/no-unused-vars': ['error']
+    }
 };

ghost/collections/package.json

@@ -32,5 +32,12 @@
         "@tryghost/in-memory-repository": "0.0.0",
         "@tryghost/tpl": "^0.1.25",
         "bson-objectid": "^2.0.4"
+    },
+    "c8": {
+        "exclude": [
+            "src/CollectionRepository.ts",
+            "src/**/*.d.ts",
+            "test/**/*.ts"
+        ]
     }
 }