Require references from typealiases to be const (#516)

This adds a language change that requires references from typealiases
to the enclosing module to be `const`.

This is required because typealiases are not late-bound.

Rationale is laid out in SPICE-0007.

Also:
* Update documentation to reflect new rules.
* Fix `Project.pkl`; mark method `const` to not break said rule.

pkl-core/src/main/java/org/pkl/core/ast/builder/SymbolTable.java

@@ -439,7 +439,7 @@ public final class SymbolTable {
         String qualifiedName,
         FrameDescriptor.Builder frameDescriptorBuilder,
         List<TypeParameter> typeParameters) {
-      super(parent, name, qualifiedName, ConstLevel.NONE, frameDescriptorBuilder, typeParameters);
+      super(parent, name, qualifiedName, ConstLevel.MODULE, frameDescriptorBuilder, typeParameters);
     }
   }