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/util/promise.ts

@@ -0,0 +1,30 @@
+/*
+ * Call functions in serial until someone resolved.
+ * @param iterable the array to iterate with.
+ * @param iteratee returns a new promise.
+ * The iteratee is invoked with three arguments: (value, index, iterable).
+ */
+export function anySeries (iterable, iteratee) {
+  let ret: Promise<any> = Promise.reject(new Error('init'))
+  iterable.forEach(function (item, idx) {
+    ret = ret.catch(e => iteratee(item, idx, iterable))
+  })
+  return ret
+}
+
+/*
+ * Call functions in serial until someone rejected.
+ * @param {Array} iterable the array to iterate with.
+ * @param {Array} iteratee returns a new promise.
+ * The iteratee is invoked with three arguments: (value, index, iterable).
+ */
+export function mapSeries (iterable, iteratee) {
+  let ret: Promise<any> = Promise.resolve('init')
+  const result = []
+  iterable.forEach(function (item, idx) {
+    ret = ret
+      .then(() => iteratee(item, idx, iterable))
+      .then(x => result.push(x))
+  })
+  return ret.then(() => result)
+}