Fix library version when used as a CMake subproject

[ktyang512]

Fixes #1119.

Problem

BTCPP_LIBRARY_VERSION used CMAKE_PROJECT_VERSION, which is the top-level
project version. When BehaviorTree.CPP was added with add_subdirectory() or
FetchContent, the library could therefore report the parent project's version.

Change

Use BehaviorTree.CPP's directory-scoped PROJECT_VERSION and add a CMake
regression that configures the library below a parent project with a different
version.

Validation

• Regression failed before the fix with BTCPP_LIBRARY_VERSION="9.8.7".
• Regression passes after the fix with BTCPP_LIBRARY_VERSION="4.10.0".
• Standalone Release build passes.
• Full serial CTest passes: 505/505.
• pre-commit run -a --show-diff-on-failure passes.
• ./run_clang_tidy.sh build-docker-clang21-ci passes in an Ubuntu 24.04
  Docker container with clangd-21 21.1.8.