feat: add transformer-remove-empty-lines

### Clear and concise description of the problem I want to remove empty lines from the code output so that the display is more compact. I intend to submit a PR for this issue. ### Suggested solution In `packages/transformers`, we could provide a `transformerRemoveEmptyLines` that filters out lines containing only whitespace. ### Alternative None considered. ### Additional context None. ### Validations - [x] Follow our [Code of Conduct](https://github.com/shikijs/shiki/blob/main/CODE_OF_CONDUCT.md) - [x] Read the [Contributing Guide](https://github.com/shikijs/shiki/blob/main/CONTRIBUTING.md). - [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.