Implement parsing observe declarations (#6674)

This implements parsing of the
[`observe`](https://docs.carbon-lang.dev/docs/design/generics/details.html#observing-a-type-implements-an-interface)
declarations.

- Added states and node kinds.
- Added node categories.
- Added a diagnostic for invalid keywords/operators.
- Implemented parser state handlers.
- Added structs to `typed_nodes.h`.
- Added parser tests.

toolchain/parse/handle_statement.cpp

@@ -4,6 +4,7 @@
 
 #include <optional>
 
+#include "toolchain/lex/token_kind.h"
 #include "toolchain/parse/context.h"
 #include "toolchain/parse/handle.h"
 
@@ -64,6 +65,7 @@ auto HandleStatement(Context& context) -> void {
     case Lex::TokenKind::Let:
     case Lex::TokenKind::Library:
     case Lex::TokenKind::Namespace:
+    case Lex::TokenKind::Observe:
     // We intentionally don't handle Package here, because `package.` can be
     // used at the start of an expression, and it's not worth disambiguating it.
     case Lex::TokenKind::Var: {