Add support for const object members (#678)

This adds support for adding the `const` modifier to object members.

Such object members are also required to have the `local` modifier applied.

This follows SPICE-0011.

pkl-core/src/test/files/LanguageSnippetTests/input-helper/modules/Birds2.pkl

@@ -0,0 +1,5 @@
+module Birds2
+
+abstract class Bird
+
+bird: Bird