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/template/output.ts

@@ -0,0 +1,17 @@
+import Value from './value'
+import { stringify } from 'src/util/underscore'
+import Template from 'src/template/template'
+import ITemplate from 'src/template/itemplate'
+import Scope from 'src/scope/scope'
+
+export default class Output extends Template implements ITemplate {
+  value: Value
+  constructor(token, strict_filters?) {
+    super(token)
+    this.value = new Value(token.value, strict_filters)
+  }
+  async render(scope: Scope) {
+    const html = await this.value.value(scope)
+    return stringify(html)
+  }
+}