Add natvis file to cmake install rules

[Ryan-rsm-McKenzie]

What does this change do?

This ensures the natvis file is installed in addition to the header files.

Is it related to an exisiting bug report or feature request?

Nope.

Pre-merge checklist

• I've read CONTRIBUTING.md
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp (how-to)
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 6 or higher
  • GCC 7 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md

Ideally the meson script would also install the natvis file, but I don't anything about meson.
For reference, (by default) this installs it into <some-install-path>/share/tomlplusplus/toml++.natvis.

[marzer]

Oh, great idea! Thanks for adding this.