Function to ask for supported image formats/extensions

[emilk]

I would like to be able to ask image what file extensions it supports give its current feature flags.

For instance: println("{:?}", image::extensions()); could print ["jpg", "jpeg", "png"] if only the image and jpeg features of image is turned on.

My specific use case for this functionality is adding file extension filters to a file dialog.

[fintelia]

I think it makes sense to expose this information. Maybe something like:

impl ImageFormat {
    pub fn reading_enabled(&self) -> bool;
    pub fn writing_enabled(&self) -> bool; 
}

Though we should probably provide a way to iterate over all the formats. I wonder if a method like fn all() -> impl Iterator<Item=ImageFormat> would be good?

[emilk]

Oh yeah, great catch on the read vs write difference. That API looks solid to me!

[fintelia]

Added in #2038