Convert image to grayscale

### Is your feature request related to a problem?

I'd like to be able to convert an `Image` to grayscale.

### Desired solution

Add method `convert_to_grayscale` to `Image` that returns the grayscale image but leaves the original unchanged.

### Possible alternatives (optional)

_No response_

### Screenshots (optional)

_No response_

### Additional Context (optional)

_No response_