[parser] More support for interfaces: methods and self deduced param. (#2427)

Summary:

Extends the current support for parsing `interface`s. In particular, adds support for parsing functions and `me` params.

toolchain/parser/testdata/generics/interface/empty.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 {
+}