Add Natvis definitions and tests for json types

[ridwanabdillahi]

This change adds Natvis visualizations for types in the json crate to help improve the debugging experience on Windows.

Natvis is a framework that can be used to specify how types should be viewed under a supported debugger, such as the Windows debugger (WinDbg) and the Visual Studio debugger.

The Rust compiler does have Natvis support for some types, but this is limited to some of the core libraries and not supported for external crates.

rust-lang/rfcs#3191 proposes adding support for embedding debugging visualizations such as Natvis in a Rust crate. This RFC has been approved, merged and implemented.

This PR adds:

• Natvis visualizations for json types.
• Tests for testing visualizers embedded in the json crate.
• A new debugger_visualizer feature for the json crate to enable the unstable debugger_visualizer Rust feature.

[gierens]

Since this project is unmaintained, I forked to https://github.com/rustadopt/jzon-rs where we for example are currently reviewing this PR and maintain the project from now on with the community. It's also available in v0.12.5 on https://crates.io/crates/jzon