chore(TypeScript): refactor objects into classes

fix: `Nil`(null, undefined) now renders as empty string
change: `parser.parseValue()` renamed to `parser.parseOutput`
change: registered tags/filters become static and shared across different liquid instances

test/e2e/express.ts

@@ -0,0 +1,97 @@
+import * as chai from 'chai'
+import * as request from 'supertest'
+import * as express from 'express'
+import * as mock from 'mock-fs'
+import Liquid from '../../dist/liquid.common.js'
+import * as chaiAsPromised from 'chai-as-promised'
+
+const expect = chai.expect
+chai.use(chaiAsPromised)
+
+describe('express()', function () {
+  var app, engine
+
+  beforeEach(function () {
+    app = express()
+    engine = new Liquid({
+      root: '/root',
+      extname: '.html'
+    })
+
+    app.set('view engine', 'html')
+    app.engine('html', engine.express())
+
+    app.get('/name', (req, res) => res.render('name', {
+      name: 'harttle'
+    }))
+    app.get('/include/:file', (req, res) => res.render('include', {
+      file: req.params.file
+    }))
+  })
+  after(function () {
+    mock.restore()
+  })
+  it('should render express views', function (done) {
+    mock({ '/views/name.html': 'My name is {{name}}.' })
+    app.set('views', ['/views'])
+    request(app).get('/name')
+      .expect('My name is harttle.')
+      .expect(200, done)
+  })
+  it('should pass error when file not found', function (done) {
+    const view = {
+      root: []
+    }
+    const file = '/not-exist.html'
+    const ctx = {}
+    engine.express().call(view, file, ctx, function (err) {
+      try {
+        expect(err.code).to.equal('ENOENT')
+        expect(err.message).to.match(/Failed to lookup/)
+        done()
+      } catch (e) {
+        done(e)
+      }
+    })
+  })
+  it('should respect root option when lookup', function (done) {
+    mock({
+      '/root/foo.html': 'foo',
+      '/views/include.html': '{% include file %}'
+    })
+    app.set('views', ['/views'])
+    request(app).get('/include/foo')
+      .expect('foo')
+      .expect(200, done)
+  })
+  it('should respect express views (Array) when lookup', function (done) {
+    mock({
+      '/views/include.html': '{% include file %}',
+      '/partials/bar.html': 'bar'
+    })
+    app.set('views', ['/views', '/partials'])
+    request(app).get('/include/bar')
+      .expect('bar')
+      .expect(200, done)
+  })
+  it('should respect express views (String) when lookup', function (done) {
+    mock({
+      '/views/include.html': '{% include file %}',
+      '/views/bar.html': 'bar'
+    })
+    app.set('views', '/views')
+    request(app).get('/include/bar')
+      .expect('bar')
+      .expect(200, done)
+  })
+  it('should respect express views (undefined) when lookup', function (done) {
+    const files = {}
+    files[process.cwd() + '/views/include.html'] = '{% include file %}'
+    files[process.cwd() + '/views/bar.html'] = 'bar'
+    mock(files)
+
+    request(app).get('/include/bar')
+      .expect('bar')
+      .expect(200, done)
+  })
+})