Default strings and vectors: Parser + Rust support (#6421)

* Fix tests.cpp

* Parser support for vector/string defaults

* tests and default empty vectors

* addressed comments

* Default strings and vectors for Rust

* Tested Rust more_defaults

* git-clang-format

* add more_defaults_test

* fixed vector default

* removed commented out code

* more unreachable

Co-authored-by: Casper Neo <cneo@google.com>

include/flatbuffers/idl.h

@@ -337,6 +337,7 @@ struct FieldDef : public Definition {
     kDefault,
   };
   Presence static MakeFieldPresence(bool optional, bool required) {
+    FLATBUFFERS_ASSERT(!(required && optional));
     // clang-format off
     return required ? FieldDef::kRequired
          : optional ? FieldDef::kOptional
@@ -973,6 +974,7 @@ class Parser : public ParserState {
   bool SupportsAdvancedUnionFeatures() const;
   bool SupportsAdvancedArrayFeatures() const;
   bool SupportsOptionalScalars() const;
+  bool SupportsDefaultVectorsAndStrings() const;
   Namespace *UniqueNamespace(Namespace *ns);
 
   FLATBUFFERS_CHECKED_ERROR RecurseError();