Restrict published files for this package (#94)

* Restrict published files for this package

An allow list (as provided by the `files` field in `package.json`)
is more effective than a deny list (as provided by `.npmignore`).

By using an allow list  we can ensure that random files don't
accidentally get published (as happened with the v2.2.0 release).

* Remove redundant mocha settings

We're no longer using mocha for testing.

Author: simonplend

.npmignore

@@ -27,9 +27,6 @@
 		"prepublish": "npm run lint && npm run test"
 	},
 	"types": "src/index.d.ts",
-	"mocha": {
-		"timeout": 15000
-	},
 	"devDependencies": {
 		"codecov": "^3.8.1",
 		"eslint": "^6.8.0",
@@ -56,5 +53,9 @@
 	"lint-staged": {
 		"*.js": "eslint --cache --fix",
 		"*.{js,css,md}": "prettier --write"
-	}
+	},
+	"files": [
+		"src/*.js",
+		"src/*.d.ts"
+	]
 }