feat: remove decimal.js and big-integer (#2379)

Reduces the bundle size by 8% (41kb)

BigInteger is now well supported so the library is no longer needed.

BigNumber was already being used by xrpl and they had almost identical
interfaces as they are both based on Java.

package.json

@@ -2,6 +2,7 @@
   "name": "xrpl.js",
   "private": true,
   "scripts": {
+    "analyze": "lerna run analyze --stream",
     "test": "lerna run test --stream",
     "test:browser": "lerna run test:browser --stream",
     "test:integration": "lerna run test:integration --stream",