Convict has Prototype Pollution via startsWith() function

Summary

### Summary A prototype pollution vulnerability exists in the latest version of the convict npm package (6.2.4). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input started with a forbidden key, it is still possible to pollute `Object.prototype` via a crafted input using `String.prototype`.

### Details The vulnerability resides in line 564 of https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js where `startsWith()` function is used to check whether user provided input contain forbidden strings.

### PoC #### Steps to reproduce 1. Install latest version of convict using `npm install` or cloning from git 2. Run the following code snippet:

```javascript String.prototype.startsWith = () => false; const convict = require('convict'); let obj = {}; const config = convict(obj); console.log({}.polluted); config.set('constructor.prototype.polluted', 'yes'); console.log({}.polluted); // prints yes -> the patch is bypassed and prototype pollution occurred ```

#### Expected behavior Prototype pollution should be prevented and {} should not gain new properties. This should be printed on the console: ``` undefined undefined OR throw an Error ```

#### Actual behavior `Object.prototype` is polluted This is printed on the console: ``` undefined yes ```

### Impact This is a prototype pollution vulnerability, which can have severe security implications depending on how convict is used by downstream applications. Any application that processes attacker-controlled input using `convict.set` may be affected. It could potentially lead to the following problems:

1. Authentication bypass 2. Denial of service 3. Remote code execution (if polluted property is passed to sinks like eval or child_process)