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/check/node_stack.h

@@ -467,6 +467,7 @@ class NodeStack {
       case Parse::NodeKind::LetInitializer:
       case Parse::NodeKind::LetIntroducer:
       case Parse::NodeKind::NamedConstraintIntroducer:
+      case Parse::NodeKind::ObserveIntroducer:
       case Parse::NodeKind::RefBindingName:
       case Parse::NodeKind::ReturnStatementStart:
       case Parse::NodeKind::StructLiteralStart:
@@ -527,6 +528,8 @@ class NodeStack {
       case Parse::NodeKind::MatchIntroducer:
       case Parse::NodeKind::MatchStatementStart:
       case Parse::NodeKind::NamespaceStart:
+      case Parse::NodeKind::ObserveEqualEqual:
+      case Parse::NodeKind::ObserveImpls:
       case Parse::NodeKind::PackageIntroducer:
       case Parse::NodeKind::ParenExprStart:
       case Parse::NodeKind::PatternListComma: