Add forward declaration header

[fktn-k]

In this PR, a header file has been added for forward declarations of this library's API class as suggested in the discussion #420.
The amalgamation scripts have been updated too so the added header file can also be used if a user choose to use the single header version of this library.

Along with the above changes, the way of defining macros which are specific to this library have been changed to make fkyaml_fwd.hpp contain minimum codes for forward declarations.
No change should appear from the user's point of view since that's only a change in the library's internal logic.

---

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[github-actions]

Upload Coverage Event Notification

Coverage data has been uploaded for the commit 510192b83e9c8ccd6ac30fc07814e26efd0c2026.
You can download the artifact which contains the same file uploaded to the Coveralls and its HTML version.

Name	fkYAML_coverage.pr422.zip
ID	2176095431
URL	https://github.com/fktn-k/fkYAML/actions/runs/11797342761/artifacts/2176095431