Adjust the augmentations grammar to enforce initializers with static final variables (#4651)

This PR updates the augmentation grammar rules such that a static final or static const variable declaration must include an initializing expression. This used to be a syntax error, it was changed because of support for augmentations, but this PR changes the approach back to make it a syntax error because the added flexibility (of being able to write new kinds of incomplete declarations) isn't needed after all now that augmentations can never "override" an implementation.