Add framework for the extern keyword. (#3755)

This doesn't add full support. I'm separating it out to make the effects
of the modifier changes clearer for review. I'm restructuring a little
with the expectation that we'll have some more categories of modifier
keywords in the future (similar to `extern`, these may not be in a "set"
such as access), and thus easily scaling up to a few more would be
useful.

toolchain/check/handle_variable.cpp

@@ -102,7 +102,8 @@ auto HandleVariableDecl(Context& context, Parse::VariableDeclId parse_node)
                        Lex::TokenKind::Var);
   auto modifiers = context.decl_state_stack().innermost().modifier_set;
   if (!!(modifiers & KeywordModifierSet::Access)) {
-    context.TODO(context.decl_state_stack().innermost().saw_access_modifier,
+    context.TODO(context.decl_state_stack().innermost().modifier_node_id(
+                     ModifierOrder::Access),
                  "access modifier");
   }