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/check/node_stack.h

@@ -463,12 +463,14 @@ class NodeStack {
       case Parse::NodeKind::ChoiceIntroducer:
       case Parse::NodeKind::CodeBlock:
       case Parse::NodeKind::ContinueStatementStart:
+      case Parse::NodeKind::CorePackageName:
       case Parse::NodeKind::ExportIntroducer:
       case Parse::NodeKind::FileEnd:
       case Parse::NodeKind::FileStart:
       case Parse::NodeKind::ForHeader:
       case Parse::NodeKind::ForHeaderStart:
       case Parse::NodeKind::ForIn:
+      case Parse::NodeKind::IdentifierPackageName:
       case Parse::NodeKind::IfConditionStart:
       case Parse::NodeKind::ImportIntroducer:
       case Parse::NodeKind::IndexExprStart:
@@ -496,7 +498,6 @@ class NodeStack {
       case Parse::NodeKind::NameQualifierWithoutParams:
       case Parse::NodeKind::NamespaceStart:
       case Parse::NodeKind::PackageIntroducer:
-      case Parse::NodeKind::PackageName:
       case Parse::NodeKind::ParenExprStart:
       case Parse::NodeKind::PatternListComma:
       case Parse::NodeKind::Placeholder: