Add --require-explicit-ids to require explicit ids (#6277)

* Add --require-explicit-ids to require explicit ids

We just got bit by a well intentioned developer forgetting that field
order by default is the field index.  3 people missed it in review.

I'm looking at ways to make it harder to mess up.  We are requesting
that developers explicitly id all fields in tables.  Automatic (opt in
for others) enforcement of this will help the effort succeed.  This
patch adds a command line flag which lets the user require ids on all
fields in tables.

* Added docs to Compiler.md as well

src/flatc.cpp

@@ -163,6 +163,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "  --reflect-types        Add minimal type reflection to code generation.\n"
     "  --reflect-names        Add minimal type/name reflection.\n"
     "  --root-type T          Select or override the default root_type\n"
+    "  --require-explicit-ids When parsing schemas, require explicit ids (id: x).\n"
     "  --force-defaults       Emit default values in binary output from JSON\n"
     "  --force-empty          When serializing from object API representation,\n"
     "                         force strings and vectors to empty rather than null.\n"
@@ -345,6 +346,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.mini_reflect = IDLOptions::kTypes;
       } else if (arg == "--reflect-names") {
         opts.mini_reflect = IDLOptions::kTypesAndNames;
+      } else if (arg == "--require-explicit-ids") {
+        opts.require_explicit_ids = true;
       } else if (arg == "--root-type") {
         if (++argi >= argc) Error("missing type following: " + arg, true);
         opts.root_type = argv[argi];