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/scope/block-mode.ts

@@ -0,0 +1,8 @@
+enum BlockMode {
+  /* store rendered html into blocks */
+  OUTPUT,
+  /* output rendered html directly */
+  STORE
+}
+
+export default BlockMode