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/node_kind.h

@@ -36,9 +36,10 @@ class NodeCategory : public Printable<NodeCategory> {
     IntConst = 1 << 8,
     Requirement = 1 << 9,
     NonExprIdentifierName = 1 << 10,
+    PackageName = 1 << 11,
     None = 0,
 
-    LLVM_MARK_AS_BITMASK_ENUM(/*LargestValue=*/NonExprIdentifierName)
+    LLVM_MARK_AS_BITMASK_ENUM(/*LargestValue=*/PackageName)
   };
 
   // Support implicit conversion so that the difference with the member enum is