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/template.ts

@@ -0,0 +1,8 @@
+import Token from 'src/parser/token'
+
+export default class Template {
+  token: Token;
+  constructor(token) {
+    this.token = token;
+  }
+}