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

src/util/assert.ts

@@ -1,6 +1,6 @@
 import { AssertionError } from './error'
 
-export default function (predicate: any, message: string) {
+export default function (predicate: any, message?: string) {
   if (!predicate) {
     message = message || `expect ${predicate} to be true`
     throw new AssertionError(message)