feat: support std::string_view in Value API

[evalon32]

This adds direct support for std::string_view when available (C++17 and above). The current API can be used with std::string_view via the low-level two-pointer methods, but is not ergonomic. E.g., compare:

Json::Value node;
std::string foo, bar, baz;
std::string_view foo_sv, bar_sv, baz_sv;

// Efficient & readable:
node[foo][bar][baz];

// Less efficient, less readable:
node[std::string(foo_sv)][std::string(bar_sv)][std::string(baz_sv)];

// Efficient, but a lot less readable:
*node.demand(foo_sv.data(), foo_sv.data() + foo_sv.size())
    ->demand(bar_sv.data(), bar_sv.data() + bar_sv.size())
    ->demand(baz_sv.data(), baz_sv.data() + baz_sv.size());

// After this change, efficient & readable:
node[foo_sv][bar_sv][baz_sv];

Change details (only with C++17 or above)

• The constructor can take a std::string_view parameter. The existing
  overloads taking const std::string& and const char* are still necessary
  to support assignment from those types.
• operator[], get(), isMember() and removeMember() take a
  std::string_view parameter. This supersedes the overloads taking
  const std::string& and const char*. The overloads taking a pair of
  pointers (begin, end) are preserved for source compatibility.
• getString() has an overload with a std::string_view output parameter.
  The one with a pair of pointers is preserved for source compatibility.