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/util/assert.ts

@@ -0,0 +1,19 @@
+import * as chai from 'chai'
+import assert from '../../../src/util/assert'
+
+const expect = chai.expect
+
+describe('assert', function () {
+  it('should not throw if predicate is truthy', function () {
+    const fn = () => assert('foo', 'bar')
+    expect(fn).to.not.throw()
+  })
+  it('should not throw if predicate is truthy', function () {
+    const fn = () => assert('', 'bar')
+    expect(fn).to.throw(/bar/)
+  })
+  it('should populate default message', function () {
+    const fn = () => assert(false)
+    expect(fn).to.throw(/expect false to be true/)
+  })
+})