Closes #3113: - Fix tokenization of `a@b: ` and `A!: ` - Add error for `a@b` (previously tokenized like `a @b`) - Extend error for `a:b` to cover `a:+`, etc. - Unify tokenization of upper- and lower-case `k: v` keys - Fix end-column value for `a: ` - Add tests as appropriate