Add opt-in support for custom string types

[mikomikotaishi]

What does this change do?

This PR adds support for custom string types through an opt-in toml::string_like<T> specialisation (inherit std::true_type to enable), as long as it can be convertible from std::string/std::string_view, and can be constructed from const std::string&/std::string_view (also std::string).

template <>
struct string_like<java::lang::String> : std::true_type {}

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

While I haven't found any issues specifically requesting support for non-std string types, I suppose #175 is a related issue. The intention is to allow custom string types to be usable with the library, however it must be opt-in (not simply inferred), to prevent things like std::filesystem::path from being detected as "string-like" (i.e. std::is_convertible_v<std::filesystem::path, std::string_view>).

Pre-merge checklist

• 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