Support `context.source` in the `parse()` reviver

[eemeli]

The JSON.parse source text access TC39 proposal became a part of JavaScript a few months ago, and its features should also be supported here.

The JSON.parse() already previously supported an optional reviver argument, a function that may transform a value as it's being parsed. This function now gets a third context argument, which (for scalar values only) has a single property .source containing a string representation of the source value.

The reviver's context argument should also be supported by this library's parse().