json_view: pass accept()'s validate-only request as a parameter

Address review of #5970:
- build() takes validate_only as an argument; the validate_only members of
  document_data and basic_json_document are removed, so neither grows
- the internal read path takes an on_error mode (discard, raise, validate)
  instead of allow_exceptions; accept() calls it with on_error::validate
- build_with() has no default for Store
- drop the unneeded static_casts in push_node()
- name the NulIsEnd argument so astyle keeps build<...> intact
- explain in the docs and code why comments/trailing commas keep the index

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/document_data.hpp

@@ -58,7 +58,6 @@ struct document_data
     std::vector<std::uint32_t> large_objects; ///< positions of the objects to index (noted while parsing)
     std::array<const char*, 4> base = {{nullptr, nullptr, nullptr, nullptr}}; ///< string bases: source, arena, edit arena, unused (indexed by flags & node_flags::storage)
     bool discarded = true;
-    bool validate_only = false; ///< accept(): build() checks the text and stores no nodes (next to `discarded`: no member moves)
 
     /// The storage of edits (editable documents only; see edit_storage.hpp).
     /// No edit moves or resizes the parsed index, and the nodes of values