add parseTimescale API

lib/parse-time-scale.js

@@ -0,0 +1,14 @@
+'use strict';
+
+const parseTimescale = str => {
+  if (typeof str !== 'string') {
+    return;
+  }
+  const str1 = str.trim();
+  const m = str1.match(/^(\d+)\s*(\w+)$/);
+  const res1 = ({1: 0, 10: 1, 100: 2})[m[1]];
+  const res2 = ({s: 0, ms: -3, us: -6, ns: -9, ps: -12, fs: -15})[m[2]];
+  return res1 + res2;
+};
+
+module.exports = parseTimescale;