Handwavy outline.

• Use Imager to load the image
• Start at the top row. Assume the width of the transparent extent for both left and right is the width of the image; similarly the top and bottom is the height.
• Walk each row and find the length of the transparent extent to the left and right
• If that length is less than the current maximum, remember it as the new current maximum
• At the same time you're going to want to look and make a similar determination for the extent in each column
• After you've walked down each row you know the width of the left most and right most transparent chunks; so crop horizontally from there over. And you should also have the vertical extents as well.