Reorganize interface parsing to be more consistent with other declarations. (#2646)

The comments in parse_node_kind.def capture the change being made here.

Before:

```
//   _external_: DeclaredName
//     InterfaceBodyStart
//     _external_: statements
//   InterfaceBodyEnd
// InterfaceDefinition
```

After:

```
//     InterfaceIntroducer
//     DeclaredName
//   InterfaceDefinitionStart
//   _external_: declarations
// InterfaceDefinition
```

Really I just want to treat introduced things consistently. `var` defines my philosophy here: it doesn't always have a `DeclaredName`, so the `VarIntroducer` _must_ be the bounding node. By being consistent with that, I believe that overall the structure becomes easier to understand (that is, there are fewer inconsistencies to understand).

This also adds InterfaceDeclaration, since I think it can be predicted we'll have that, and it's helpful for making recover consistent with HandleDeclarationError.

Similarly, I'm also trying to standardize the loop processing a little with HandleDeclarationLoop. In the current approach, InterfaceDefinitionFinish isn't a necessary state, so I'm removing it.

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

@@ -0,0 +1,13 @@
+// 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: 'InterfaceIntroducer', text: 'interface'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'Foo'},
+// CHECK:STDOUT: {kind: 'InterfaceDeclaration', text: ';', subtree_size: 3},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+interface Foo;