Add Natvis visualizations for the some widestring types

[ridwanabdillahi]

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

Debugging types such as the U16String type under a Windows debugger, (WinDbg or the Visual Studio debugger), the actual string is not shown at first glance. This takes digging through the internal type and enforcing conversions to see the actual data. 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.

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.

This PR adds:

• Natvis visualizations for core types in the widestring crate such as types defined in the ucstring, ustring and utfstring modules.

• Tests for testing visualizers embedded in the widestring crate.

• Updates to the CI pipeline to ensure tests for visualizers are run so they do not break silently.

• Updates to the Makefile.toml to enable debuginfo when running debugger_visualizer tests since they will need debugging info and symbols to be present.

• A new debugger_visualizer feature for the widestring crate to enable the unstable debugger_visualizer feature.