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/parser/output-token.ts

@@ -0,0 +1,8 @@
+import DelimitedToken from './delimited-token'
+
+export default class OutputToken extends DelimitedToken {
+  constructor(raw, pos, input, file, line) {
+    super(raw, pos, input, file, line)
+    this.type = 'output'
+  }
+}