Make sure decorators are on their own line, and don't stop highlighting at
the first dot when they are imported, e.g.
```
import enum
@enum.unique
class A(enum.Enum):
…
```
Ideally highlighting shouldn't stop at the first parenthesis either
(e.g. `@foo(['1'])`), but the current code somewhat highlights the contents
of the parens already, which is good enough in most cases.
