Previously we would treat the empty string as `null`. This caused
JavaScript like this to fail:
```js
var object = {};
try {
object = JSON.parse("");
} catch {}
var array = object.array || [];
```
Since `JSON.parse("")` returned null instead of throwing, it would set
`object` to null and then try and use it instead of using the default
backup value.
Parse JSON floating point literals properly,
No longer throwing a SyntaxError when the decimal portion
of the number exceeds the capacity of u32.
Added tests to AK/TestJSON and LibJS/builtins/JSON/JSON.parse
The current implementation of step 2a sort of manually implemented GetV
with a ToObject + Get combo. But in the call to Get, the receiver wasn't
the correct object. So when invoking toJSON, the receiver was an Object
type rather than a BigInt.
This also adds spec comments to SerializeJSONProperty.
