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>

tests/more_defaults.fbs

@@ -0,0 +1,7 @@
+
+table MoreDefaults {
+  ints: [int] = [];
+  floats: [float] = [     ];
+  empty_string: string = "";
+  some_string: string = "some";
+}