feat: add IWYU pragmas for public/private headers (#1386)

[harleywilsoneng]

Summary

Adds Include What You Use pragmas across yaml-cpp public headers, following the same pattern as googletest:

• yaml-cpp/yaml.h marks its transitive includes with // IWYU pragma: export
• Internal headers mark // IWYU pragma: private, include "yaml-cpp/yaml.h" plus friend "yaml-cpp/.*"

This lets IWYU redirect #include "yaml-cpp/node/node.h" to the umbrella header so users get impl.h and other required definitions, avoiding link errors.

Fixes #1386.

Test plan

• Headers compile unchanged (comments only)
• Run IWYU on a sample TU using YAML::Node and confirm it suggests yaml-cpp/yaml.h

[SGSSGene]

Thank you very much for this contribution!
Personally not using IWYU but pretty sure people would love to use it.

(Also reminds me, we should remove the code for #pragma once, that creates some noise in the files).