Add a LevelEnabled method to the ZapEventLogger to more conveniently check if a log level is enabled for the logger.

This is an alternative to calling:
```
    logger.Level().Enabled(zapcore.Level(LevelDebug))
```
resulting in the simpler and more readable call to:
```
    logger.LevelEnabled(LevelDebug)
```

This has the added bebefid of not requiring an import of `"go.uber.org/zap/zapcore"`