Implement #4864: Core is a keyword (#4909)

Change representation of package names from `IdentifierId` to
`PackageNameId`, and add a special value `PackageNameId::Core` for the
Core package. Add a `Core` expression to name the Core package, and
support for parsing the `Core` keyword in `package` and `import`
declarations.

For now, I've made no changes to instruction fingerprinting or name
mangling. This means that fingerprints and mangled names will collide
between names in the `Core` package and names in a `r#Core` package. See
#4908.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/parse/handle_expr.cpp

@@ -165,6 +165,11 @@ auto HandleExprInPostfix(Context& context) -> void {
       context.PushState(state);
       break;
     }
+    case Lex::TokenKind::Core: {
+      context.AddLeafNode(NodeKind::CoreNameExpr, context.Consume());
+      context.PushState(state);
+      break;
+    }
     case Lex::TokenKind::SelfValueIdentifier: {
       context.AddLeafNode(NodeKind::SelfValueNameExpr, context.Consume());
       context.PushState(state);