Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practices as demonstrated in the repository.

Description

Old archives of Windows icons from software found on the Internet archive have an .ICN extension. When converting them to BMP I get this error message:

convert 007.ICN ../tmp.bmp
convert-im6.q16: no decode delegate for this image format ICN' @ error/constitute.c/ReadImage/581.
convert-im6.q16: no images defined ../tmp.bmp' @ error/convert.c/ConvertImageCommand/3234.

So I added ICN to the Windows Icon loader and tested it with the attached icons:
HR7ICN.zip