Add export keyword handling. (#3944)

This provides `export import` logic in lex, parse, and check; `export
name` logic is only in lex and parse, not check.

I think with `export name` I'm going to need to modify import_ref and
some consolidation logic, whereas `export import` seems feasible to keep
as primarily import logic. Given the implementations were looking like
they'd diverge more substantially, I thought it'd be helpful to cut the
PR here.

toolchain/check/handle_export.cpp

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/context.h"
+#include "toolchain/parse/typed_nodes.h"
+
+namespace Carbon::Check {
+
+auto HandleExportIntroducer(Context& /*context*/,
+                            Parse::ExportIntroducerId /*node_id*/) -> bool {
+  return true;
+}
+
+auto HandleExportDirective(Context& context, Parse::ExportDirectiveId node_id)
+    -> bool {
+  return context.TODO(node_id, "ExportDirective");
+}
+
+}  // namespace Carbon::Check