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_decl_scope_loop.cpp

@@ -8,6 +8,7 @@
 #include "toolchain/parse/context.h"
 #include "toolchain/parse/handle.h"
 #include "toolchain/parse/node_kind.h"
+#include "toolchain/parse/state.h"
 
 namespace Carbon::Parse {
 
@@ -130,6 +131,8 @@ static constexpr auto DeclIntroducers = [] {
       StateKind::TypeAfterIntroducerAsInterface);
   set(Lex::TokenKind::Namespace, NodeKind::NamespaceStart,
       StateKind::Namespace);
+  set(Lex::TokenKind::Observe, NodeKind::ObserveIntroducer,
+      StateKind::ObserveAfterIntroducer);
   set(Lex::TokenKind::Require, NodeKind::RequireIntroducer,
       StateKind::RequireAfterIntroducer);
   set_contextual(Lex::TokenKind::Let, RegularContext, NodeKind::LetIntroducer,