chore(TypeScript): ship Liquid to class

BREAKING CHANGE: calling `Liquid()` without `new` now becomes invalid

demo/express/app.js

@@ -1,8 +1,8 @@
 const express = require('express')
 const Liquid = require('../..')
 
-let app = express()
-let engine = Liquid({
+const app = express()
+const engine = new Liquid({
   root: __dirname, // for layouts and partials
   extname: '.liquid'
 })
@@ -12,7 +12,7 @@ app.set('views', ['./partials', './views']) // specify the views directory
 app.set('view engine', 'liquid') // set to default
 
 app.get('/', function (req, res) {
-  let todos = ['fork and clone', 'make it better', 'make a pull request']
+  const todos = ['fork and clone', 'make it better', 'make a pull request']
   res.render('todolist', {
     todos: todos,
     title: 'Welcome to liquidjs!'