Feature proposal: add color parameter to `[!code highlight]`

### Clear and concise description of the problem I want to be able to pass a color parameter to `[!code highlight]` so that a code block can have multiple lines highlighted with different colors. ### Suggested solution ```js console.log(`hello`) // default highlight // [!code highlight] console.log(`world`) // blue highlight // [!code highlight:#abc] const sum = 1 + 1 // green highlight // [!code highlight:#bca] const fn = () => 1 + 1 // purple highlight // [!code highlight:#bac] ``` ### Alternative _No response_ ### Additional context _No response_ ### 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.