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/unit/xhr.ts

@@ -0,0 +1,45 @@
+import { read } from 'src/parser/template-browser'
+import * as sinon from 'sinon'
+import * as chai from 'chai'
+
+chai.use(require('chai-as-promised'))
+
+const expect = chai.expect
+
+describe('template-browser', () => {
+  if (+process.version.match(/^v(\d+)/)[1] < 8) {
+    console.info('jsdom not supported, skipping xhr...')
+    return
+  }
+  let server
+  beforeEach(() => {
+    server = sinon.createFakeServer()
+    server.autoRespond = true
+    server.respondWith('GET', 'https://example.com/views/hello.html',
+      [200, { 'Content-Type': 'text/plain' }, 'hello {{name}}']);
+    (global as any).XMLHttpRequest = sinon.useFakeXMLHttpRequest()
+  })
+  afterEach(() => {
+    server.restore()
+    delete (global as any).XMLHttpRequest
+  })
+  describe('#read()', () => {
+    it('should get corresponding text', () => {
+      return expect(read('https://example.com/views/hello.html'))
+        .to.eventually.equal('hello {{name}}')
+    })
+    it('should throw 404', () => {
+      return expect(read('https://example.com/not/exist.html'))
+        .to.be.rejectedWith('Not Found')
+    })
+    it('should throw error', function (done) {
+      read('https://example.com/views/hello.html')
+        .then(() => done('should not be resolved'))
+        .catch(function (e) {
+          expect(e.message).to.equal('An error occurred whilst receiving the response.')
+          done()
+        })
+      server.requests[0].error()
+    })
+  })
+})