[parser] Start re-implementing interfaces using the stack parser. (#2412)

First change towards re-implementing interfaces using the new parser. I kept it small to make sure we are on the same page regarding stack states and how the parse tree should look like.

Co-authored-by: ergawy <kareem.ergawy@guardsquare.com>

toolchain/parser/testdata/generics/interface/basic.carbon

@@ -0,0 +1,15 @@
+// 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
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
+// CHECK:STDOUT:     {kind: 'InterfaceBodyStart', text: '{'},
+// CHECK:STDOUT:   {kind: 'InterfaceBody', text: '}', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: 'interface', subtree_size: 4},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+interface foo {
+}