Set minimum node version to 10.22; specify v12 for nvm (#100)

- Using older versions of node can result in this error:

    TypeError: buf.writeBigUInt64BE is not a function

- nvm can automatically use the version specified in .nvmrc. Use node
  version 12 because it is currently recommended for most users

packages/ripple-binary-codec/.nvmrc

@@ -1 +1 @@
-10.16
+12

packages/ripple-binary-codec/package.json

@@ -49,5 +49,9 @@
   "readmeFilename": "README.md",
   "prettier": {
     "semi": true
+  },
+  "engines": {
+    "node": ">=10.22",
+    "yarn": "^1.22.4"
   }
 }