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/template.ts

@@ -0,0 +1,27 @@
+import { resolve } from '../../src/parser/template'
+import * as mock from 'mock-fs'
+import * as chai from 'chai'
+import * as path from 'path'
+import * as chaiAsPromised from 'chai-as-promised'
+
+const expect = chai.expect
+chai.use(chaiAsPromised)
+
+describe('template', function () {
+  before(function () {
+    mock({
+      '/foo/bar.html': 'bar'
+    })
+  })
+  describe('#resolve()', function () {
+    it('should resolve based on root', function () {
+      const filepath = resolve('bar.html', '/foo', { root: [] })
+      const expected = path.resolve('/foo/bar.html')
+      return expect(filepath).to.eventually.equal(expected)
+    })
+    it('should resolve based on root', function () {
+      return expect(resolve('foo.html', '/foo', { root: [] }))
+        .to.rejectedWith(/Failed to lookup foo.html in: \/foo/)
+    })
+  })
+})