Unify handling of string-like arguments across the API

[legendecas]

Follow-up to #1741 / #1742.

Several APIs still carry multiple overloads for different string types. Now that the library requires C++17, we can modernize by preferring std::string_view and removing the redundant UTF-8 overloads, replacing them with a single constrained template, as a semver-major change.

For String::New, Symbol::New, and Symbol::For, collapse the const std::string& + std::string_view pair into:

template <typename T,
          std::enable_if_t<
              std::is_convertible_v<const T&, std::string_view> &&
              !std::is_convertible_v<const T&, const char*>, int> = 0>
static String New(napi_env env, const T& t);

This matches the C++17 standard-library idiom (std::string's find/append/compare/… members). The !is_convertible_v<const T&, const char*> clause keeps const char* and literals on the dedicated overload and filters nullptr (avoiding a UB string_view(nullptr)).

The enable_if alias is shareable across APIs e.g. enable_if_string_view_like_t<T>.