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/html-token.ts

@@ -0,0 +1,9 @@
+import Token from './token'
+
+export default class HTMLToken extends Token {
+  constructor(str, begin, input, file, line) {
+    super(str, begin, input, file, line)
+    this.type = 'html'
+    this.value = str
+  }
+}