let print_to_stream also print the end of a source region

[N-Dekker]

With this commit, std::cout << node.source() should print both the begin and the end of the source region.

• 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 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

[N-Dekker]

@marzer Thank you for the suggestion! Shall I squash you suggested commit on my fork? Otherwise you may also do a squash-and-merge on GitHub when you think this PR is good enough, of course.

[marzer]

All good, we can squash-and-merge when it is brought into main. Just need to update the single-header version and I'll merge it in 👍🏼

[marzer]

Thanks 🎉