Allow tag_invoke to throw without std::terminate (#2858) - #2872

A user tag_invoke customization is allowed to throw, but the exception
could never reach the caller: every simdjson_result<...>::get<T>()
wrapper was unconditionally noexcept, so a throw hit std::terminate.

- New concept nothrow_gettable<T, ValT>: true when there is no custom
  tag_invoke for T or that tag_invoke is noexcept. get<T>() and
  get<T>(T&) on value, array, object, document, document_reference and
  all their simdjson_result<...> wrappers are noexcept(nothrow_gettable).
  Built-in types and noexcept customizations keep every layer noexcept.
  document_reference::get was checking the document customization; fixed.
- Allocating customizations (containers, maps, unique_ptr, shared_ptr,
  reflected structs) are noexcept(false) so bad_alloc and user
  exceptions propagate, and so that the exception specification of
  get<T>() never recurses into T for self-referential types.
- The appendable-container loop always deserializes into a temporary and
  moves it in: master's "in place" path already went through a
  simdjson_result<T> temporary, a plain temporary is ~3% faster on the
  citm static-reflection benchmark, and it needs no cleanup on error or
  exception (the container keeps exactly the successfully parsed
  elements).
- unique_ptr/shared_ptr customizations own the allocation before calling
  get(), so a throw cannot leak and the caller's pointer is unchanged.
- Static reflection: exceptions propagate out of reflected structs too.
  The reflected-struct tag_invoke also matched simdjson's own types
  (array, object, number, document, ...), conflicting with their
  explicit noexcept get<T>() specializations; they are excluded now.
  With reflection enabled get<T>() is noexcept(false) for every struct
  T, so a legacy explicit specialization
      template<> simdjson_result<Car> value::get() noexcept { ... }
  no longer matches the primary template; such specializations are
  redundant under reflection and the two tests carrying them guard them
  with !SIMDJSON_STATIC_REFLECTION. Documented in basics.md.
- New tests/ondemand/ondemand_tag_invoke_exception_tests.cpp.