Avoid using Symbol.for('ok-computer.object-root')

[richardscarrott]

In many cases, you want to be able to serialize errors (usually w/ JSON.stringify), however currently the object root property is lost as it's a symbol.

  const user = object({ name: string });
  const error = user({ name: 123, foo: 'not allowed' });

  console.log(error);
  // {
  //  name: 'Expected typeof string',
  //  [Symbol(ok-computer.object-root)]: 'Unknown properties "foo"'
  // }

  console.log(JSON.stringify(error));
  // { "name": "Expected typeof string" }

It was originally a string (__root) but was changed to a symbol to avoid clashes with user properties. In hindsight I think it's worth going back to a string property and a) giving it a better namespace and b) throwing if a validator is passed in with a conflicting name.

e.g.

const user = object({ name: string });
const error = user({ name: 123, foo: 'not allowed' });

 console.log(error);
 // {
 //  name: 'Expected typeof string',
 //  'ok-computer.error': 'Unknown properties "foo"'
 // }

console.log(JSON.stringify(error));
// { "name": "Expected typeof string" }
  
object({ 'ok-computer.error': string }); // throw

NOTE: As a reminder, we return a "root" error as a property like this (rather than { name?: string } | string), so the returned interface is always an object making it easier to consume).

Implementation details

1. This change may mean listErrors no longer needs to use ownEnumerableEntries https://github.com/richardscarrott/ok-computer/blob/master/src/ok-computer.ts#L76
2. May want to avoid calling it 'root' now it's used for both "Expected object" and "Unknown properties foo"
3. The same needs to be done for array; although arrays obviously don't non-index properties...