meson: install header and pkg-config file and in subprojects

[stephanlachnit]

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What does this change do?

Install the headers and the pkg-config file also in Meson subprojects. The reasoning behind this is that if you install a Meson project using TOML++ (potentially into a non-standard location like /opt/project), and also create a pkg-config file for your project, TOML++ might get added as public or private dependency. In that case, the header (and pkg-config file) should be installed.

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

No

Pre-merge checklist

• [x] I've read CONTRIBUTING.md
• [x] 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 8 or higher
  • [ ] GCC 8 or higher
  • [ ] MSVC 19.20 (Visual Studio 2019) or higher
• [ ] I've added my name to the list of contributors in README.md